Did you recently upgrade your Moodle site? I had the same issue when I first upgraded one of my Moodle sites to Moodle 3.7. Turns out that particular site was using a theme based on the now deprecated BootstrapBase base theme. Instead of telling me what the problem was, Moodle simply would not let me log in.
To resolve the issue, I renamed my theme's directory to *.old. Not being able to find the theme any more, Moodle defaulted back to the Boost theme and I was able to now login.
I can't say it always works but I have had some good success in temporarily renaming a directory by adding an extension to it such as .old as Moodle ignores these. You would not get the same result if you added _old to the folder name.
You should be able to do this with most plugins to temporarily diablo them just don't forget to rename it back afterwards.
Hoe you find this useful.