Hi Pieter,
I ran into the same problem today and it turned out to be the Logging module. I had set the log level to TRACE, which seems to put too much pressure on the database. Setting the log level to a different level isn't possible, the app always seems to go back to log level TRACE. Removing the Logging module has resolved this issue for me, though this is not a permanent fix.
I'm going to keep trying to fix this. If I find a solution I'll let you know!
Edit:
I think I have found the issue, it has something to do with the constant 'Level'. See this forum post for more info: https://forum.mendix.com/questions/6772/Use-of-constants-and-the-Sandbox-environment
I was able to set the logging to NONE, but anything else than that doesn't seem to work.
Guess there is nothing left to do but wait for a fix.