I guess I should have been more specific when I said “Try changing the decimal seperator settiing.”… 
Glad you figured it out. I’m a bit baffled though why it makes a difference for you as your locale decimal point is the “period” .
so it shouldn’t matter if the setting is checked or unchecked… as “unchecked” means use a period, where “checked” means use a period. I’m wondering if I should change the pref UI to just use a radio button with the choices Period and Comma. Hmmmm.
HeyTheo… just figured I’d give you an update. I’ve been working on migrating Seamly to Qt6, and one of the things I’ve had to fix is the way the app deals with the locale separators - which has been a source of the QTextCursor errors.
The one thing I learned is that some locales use UTF-16 rather than UTF-8, which explains why the app can’t use certain locales as the encoding for the decimal & group separtors is different leading to the cursor errors. It would explain why the en_GB locale - which I believe would be your user locale - doesn’t work, while my en_US does. When the Decimal separatior pref is unchecked, the app uses the standard C locale… which in my case is the same for the User locale, but not for you.
In QT6 they changed the return type for the locale decimal & group separators, as well as the plus and minus signs from a QChar to a QString - except the math parser expects a char. So… in a nutshell in fixing the math parser to use the QString separators, I believe it also fixed the QTextCursor issue. I have been able to run a Windows QT6 build with the locale set to en_GB with no QTextCursor errors. Once I get a QT6 branch to build maybe you could install a MacOs artifact build and see if indeed the QTextCursor will no longer be an issue.
Thank you for the update! That makes sense. I would certainly be happy to test that out for you, once it’s ready drop me a link to the artifact build