Is it still a "Error: Failed opening required" message that you're seeing?
If so, this sounds like a permissions or corrupted/faulty file issue potentially. Especially if clearing the datastore for a period of time is clearing the issue then an action on your server presents itself and the error triggers again. This could be an issue with your storage device, reading/writing to that storage device, permissions, etc...
While not really a long-term solution, you can temporarily switch your datastore to be stored in the database to see if this helps resolve this issue. If it does, it would be something you need to talk with your hosting provider about.
To switch your datastore to database, go to ACP -> System -> Advanced Configuration -> Data storage.