Can't join my server because of "Invalid player data"

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    So today I made a server with the MineFortress mod. I added the right forge version and the right api version. In my Minecraft client I'm using the same mods and versions(I also added optifine but without optifine is still not working).

    I tried to delete the player data too, but didn't helped at all.

    If MineFortress is the only mod installed, conatact the mod author about it