Problem/Bug Player's levels reset after relogging, 1.12.2

Whenever one of my players relog, it resets their EXP level. I tried it myself as OP and I had no problems, but when I moved myself to default rank and deopped myself I had the same problem..

I've never experienced this before and it's seriously annoying for my players, any help would be seriously appreciated!

Also, here's the log, and here's a picture of my plugins.