Valid tokens stop being accepted

[Jojo-IO]

It occured twice in the last days that I issued a token which was valid and accepted by the server, and from the next day on it says "Decoding process has failed, token is either malformed or signature is invalid". Testing the tokens with other tools, they are still valid (expiring in one year).
I would like to know if anybody could think of an idea, how this could be theoretically possible. There were no changes made on the server.

[AnthonyDeroche]

I have no clue on how to solve this issue, it might need deeper investigation.