FATAL ERROR: gui_theme_t::init_gui_themes - Wrong theme loaded
This has been reported quite a few times. The weird thing is that I could swear I played yesterday and it worked, from the same installation. After getting this error I downloaded a fresh one (the zip file from the "How to download Simutrans" page) but to no avail. Deleting settings-extended.xml did the trick, though.
Now that I think of it, the only thing I never did before yesterday was play around in-game with themes and font settings. I don't think I messed with any files, though.
Is it possible that theme settings are saved but then the settings file gets corrupted somehow? Mine looks OK, i.e. not garbled, but I wouldn't recognize syntax errors or something like that.
Thank you for the report. It is difficult to look into this without being able to reproduce this reliably. If you or anyone else could document steps necesary to reproduce this, that would be most helpful.