A signature error will most likely occur if the expected algorithm is not used. For example, the application may expect RS256, but you might have configured HS256.
In my private cloud, I have encountered the same error. I had to change the setting to RS256 multiple times to resolve the issue.
You can find the settings under:
Application Settings → Advanced Settings → OAuth → JSON Web Token Signature Algorithm
Please share the entire error message; it will be helpful in identifying the issue.