I’m the person who added the Filen backend and a Filen employee so I can help out here. Though I don’t really use this forum so the best way to get a reply from me is to make a GitHub issue and mention me.
These are three distinct issues.
@LukePet you seem to have copied the API key incorrectly (or not at all), that is likely the cause of your issue.
@l3m0r I suspect you misstyped your password. There was someone else who had an issue with pasting their password on a specific OS not working as expected. If you’re 100% sure that’s not the issue, try changing your Filen password (you should be able to change it to the same thing), to regenerate the master key string
@Aliases This one seems more complicated, if you recreate the config, do you still get the exact same issue? Do you have any issues when looking at the directories in the root of your account on the website or any other client (the name of files or folders being NOT_DECRYPTED or something like that)?
ETA: @Aliases If you could open a support ticket (on the Filen site) with information about your account the support team would better served to debug your specific issue.
ETA2: @LukePet or you may have changed your password which also changed your API key