Hi,
SAP support found the cause of this behaviour. The SMP Service user is generating temporary files in its windows user local data (C:\Users\smpServiceUser\AppData\Local\Temp) and a limit was reached.
Only deleting a few of these files solved the issue, at least for a while. I'm still trying to investigate how can this issue be avoided in the future. I think SAP should archive/delete this files automatically after a while but I'm not sure if it can yet.