Martin,
Your issue is numerical instability. You will need to adjust the options to NDSolve. Try a stiff solver. Also look at some of the PDE solver options such as Method of Lines. Let me know what you try and when I have some time I will experiment a bit.
Regards,
Neil
Maybe this helps.
Maple 2018 can't find symbolic solution,only a general.
See attached file and Comparison
Regards,MI
Attachments:
