An option (“DebuggerSettingsBreakpointsWindowSize->{Full, Full}”) was encountered with a right-hand side value out of the supported range for this option.
Go into the Option Inspector (the Format >Option Inspector… menu item) and set the scope to Global. Then in the search box type in DebuggerSettings
A bunch of sub-options under DebuggerSettings will appear. Next to the BreakpointsWindowSize sub-option there is a pulldown menu. Select Automatic under this menu to reset things. Quit Mathematica and restart. See if this corrects the issue.
Hi David-that fixed it. Many thanks for the prompt response.