@sterndata
I am not sure where to look for this log, but I did find a solution that seem to fix the issue: Appearance –> Widgets —> Screen Options—> Enable Accessibility Mode.
Maybe my version is too old, but in accessibility Mode the text widget saves.
Thank you anyway for responding.
Just for my own interest, why are you not current on WordPress?
Official support for the theme I use has discontinued, and even when it was available, every WP update used to break something. I am satisfied with my version. No money change hands on my site, so I doubt it is attractive for hackers.
