I came across a similar issue during the beta test for version 12. There were some old components that earlier versions of Mathematica used that were not removed during a simple installation.

So, now, I routinely remove the $UserBaseDirectory (etc. -- depends on your installation history) before any upgrade. You should move this directory (folder) to another location so that you can copy back any customizations you have installed, such as stylesheets or applications or packages.

Since I did this, there have been no issues, and macOS Mojave reports that not only is Mathematica fully 64 bit, there are no legacy apps or components associated with Mathematica.

There is a new web page (which Dale has linked to) about uninstalling Mathematica. This is not the "official" web page, and recommends the use of a different application.

The official support page:

Uninstalling Mathematica

refers to $BaseDirectory, which no longer exists for most users, but is otherwise pretty thorough. This is the method that I use. You will need to re-enter the activation key (or restore the file that contains it), and then copy back any stylesheets, etc. that you want to keep.

Once you have gotten rid of the "cruft", you can probably install new versions by simply replacing the app. I did this for the final version of Mathematica 12, and everything worked just fine.

Note that this is probably a general issue for upgrades of many apps, that there may be older bits that are no longer used, but which are still on disk. The problem is more obvious now due to the 64 bit issue. (I have had to do a similar thing to other apps that I have upgraded over multiple versions of macOS and multiple computers.)

Further, if you encounter any weirdness with Mathematica, I would recommend un-install and re-install including removing the $UserBaseDirectory. If nothing else, it will restore your installation to a "known state" for dealing with tech support.

I checked my interactions with Apple and Wolfram re Mojave. I was a beta tester for Mojave on my 'spare' computer (a 13 inch MacBook Pro). I tested each beta with Mathematica 11.3 -- actually Wolfram Desktop from a Wolfram|One trial, since I don't have a third license. I had no issues with the program itself, although there were issues with the download manager.

When I installed macOS Mojave on my two main computers (a 2017 27 inch iMac and a 2016 15 inch MacBook Pro), there were issues with the installation. I ran disk utility immediately after the installation, and it found orphaned dream object. Apple support said that these were left-overs from the installation, and they should go away. I know how entropy works, though.

I ended up using recovery mode (cmd-R on restart) to erase the hard drive, re-install Mojave, and then restore from a back-up. On my MacBook Pro, I saved the contents of my documents folder and did a clean install.

After I did that, there were no more problems with Disk Utility. In addition, inside the ~/Library folder, there was a lot less stuff from Wolfram. The download managers, and some other installers apparently do not clean up after themselves very well.

These computers had been updated from older installations over the years, and so there was a lot of cruft in the System Folder (and elsewhere). Depending on your history, the same may be true for you. It is possible that your issues have to do with obsolete stuff in the $UserBaseDirectory.

If you have no issues with other apps, or with Disk Utility, you may simply want to remove the Mathematica folder from within ~/Library (aka $UserBaseDirectory). When you restart Mathematica, you will need to re-authorize. I would keep the old folder somewhere, especially if you have custom stylesheets, palettes, or other add-ons.

Since it is only an issue of download time (unless you saved the installer), if you have invested this much time, it makes sense to install a fresh copy of Mathematica. Note that doing this without setting the $UserBaseDirectory to its 'new state' is a waste of time in most cases.

NOTE that on macOS, the $BaseDirectory should not exist. It was deleted a couple of macOS upgrades ago for security reasons, although the Wolfram documentation and FAQ still refer to it. It is inside the system's Library folder, as opposed to the User's Library folder. If you have a Mathematica folder there, you should probably remove it. Mine disappeared with the OS update automatically. Wolfram Support will tell you that you can manually copy the Mathematica folder from your User library there, but in my opinion, that is asking for trouble.

In the 'dark ages', when the OS came on DVD-ROM, I would always do an erase and install whenever there was a new OS. It is a lot more convenient to simply upgrade in place, but there is a price. I found stuff from more than 10 years ago inside my system folder that really should not have been there. My MacBook Pro seems zippier, but that could just be an impression. I really had few, if any problems before, but I did recover significant disk space. I also managed to lose preferences, etc. for software that I no longer use.

I am puzzled... Maybe it has to do with MacBookPro with solid-state drive. Is my Mathematica version corrupted?

Update:

The expression does not simplify even after re-downloading Mathematica 11.3 and reinstalling under Mojave. Never heard of version 11.4. Not available for download.

I hav the same (correct) result. No issue with Mojave so far.

Does anyone have information about whether Mathematica 10.0.2 works fine on Mojave?

It is very unlikely that this has anything to do with Mojave. Have you tried restarting Mathematica? OS upgrades would typically cause problems with the front end only.

this formula gives the following answer: 1 - (Q^2 r^2)/6 + (Q^4 r^4)/120

No problem here

I've switched to Mojave and now the Front End doesn't open. The app runs in background but there is no way to use it (except from a Terminal).

I've reinstalled 11.3.0 on the Mac but the problem persists.

Does anybody have an idea about what the problem could be? Thanks.

An analysis on the process shows:

2018-10-01 17:13:55.066 sample[26384:827987] Failed to create CSSymbolicatorRef for pid 26375 Mathematica

Sampling process 26375 for 3 seconds with 1 millisecond of run time between samples

Sampling completed, processing symbols...

Analysis of sampling Mathematica (pid 26375) every 1 millisecond

Process: Mathematica [26375]

Path: /Applications/Mathematica.app/Contents/MacOS/Mathematica

Load Address: 0x1000

Identifier: com.wolfram.Mathematica

Version: ???

Code Type: X86

Parent Process: ??? [1]

Date/Time: 2018-10-01 17:13:55.011 +0100

Launch Time: 2018-10-01 17:13:49.876 +0100

OS Version: Mac OS X 10.14 (18A391)

Report Version: 7

Analysis Tool: /usr/bin/sample

Physical footprint: 8K

Physical footprint (peak): 8K

Call graph:

2677 Thread_827531: Main Thread   DispatchQueue_<multiple>

  2677 ???  (in <unknown binary>)  [0xcfc000]

Total number in stack (recursive counted multiple, when >=5):

Sort by top of stack, same collapsed (when >= 5):

    ???  (in <unknown binary>)  [0xcfc000]        2677

Binary images description not available

Sample analysis of process 26375 written to file /dev/stdout

Hello, Thanks for your reply. I've tried your suggestions, twice, but I still have the same problem. I'll contact Support. Thank you.

Other apps do see the same message, notably Adobe Application Manager which is part of Creative Cloud. I've heard that under Mojave the warning repeats every thirty days. Under High Sierra it was indeed first time only.

Mathematica 11.3 does officially support macOS Mojave, as documented here.

Mathematica 11.3 has a 32 bit Front end. Mojave runs 32 bit apps, but gives you a warning. The next macOS will not run 32 bit apps.

The next release of Mathematica will surely be 64 bit -- as shown in Stephen Wolfram's Twitch sessions -- so other than the warning (which appears just once, I believe), we are ok.