Error message persists despite uninstalling and reinstalling Studio: the input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters" error when val

This happened suddenly, not after an obvious update and is persisting despite uninstalling and reinstalling Studio and Studio-related add-ons. 

The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters" error when validating the SAML2P Identity Provider


I am happy to remove all Trados related data, which reinstallation doesn't seem to do -- as I saved my project files elsewhere already -- but I really need  this sorted asap because I've already lost 2 days trying to find different fixes.

Running Freelance Studio 2022 on a subscription basis. 

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