Hi everyone, I had this problem too. I share the solution I used
Note that ps_mbo is required for the proper working of Prestashop Checkouto module, so I recommend keeping it active.
1. disable ps_mbo module
2. open filemanager (now works)
3. activate ps_mbo
4. open filmanager again (still works)
5. ATTENTION! repeat by logging in with each administrator accounts
Please note: Cache operations have no impact.