JWT Authentication for WP-API and error

Hello,
I was using WP API with JWT Authentication for WP-API and everything worked well. Out of the nowhere maybe after 2 months since last usage the application started to throw this error:
System.AggregateException: One or more errors occurred. (Unexpected character encountered while parsing value: <. Path ”, line 0, position 0.)

The test instance works without problem.

I tried to reinstall the plugin and change the configuration in .htaccess and wp-config as well, but still the problem is here. Everything is up to date. The JWT auth. version 1.3.2 and wordpress 6.1.1.

Any ideas where can be the problem? Thanks!