I also can't save any settings in WPML. For example, I want to make a CPT translatable, but when I check this off, it doesn't save the setting.
Solution:
The client reported the following on how the issue was solved.
I have tested deactivating theme and plugins and now activated them again. The error is gone. The problem was the PHP version - reverted to 8.0, and the error is gone.
**** Important! Please make a full site backup (files and DB) before you proceed with those steps****
For your information, we have a few known issues with PHP version 8+. You can read about them here.