"Internal server error" in OMERO 5.5.0


today I noticed our OMERO server was not reachable and just displayed "An internal server error occurred. Please try again later. "
When I looked at the Blitz.log if found several Exceptions:

serverExceptionClass = "ome.conditions.DatabaseBusyException"
 message = "cannot create transaction"

2019-07-05 08:53:47,213 ERROR [                      omero.cmd.SessionI] (erver-6458) Error destroying servant: 222b3673-bbab-449d-81d3-a1d3a3ebb09fomero.api.RawFileStore=ome.services.blitz.impl.RawFileStoreI@499b5988

2019-07-05 08:53:47,208 INFO  [        ome.services.util.ServiceHandler] (erver-6458)  Excp:    org.springframework.transaction.CannotCreateTransactionException: Could not open JDBC Connection for transaction; nested exception is ome.conditions.DatabaseBusyException: Cannot acquire connection

2019-07-05 08:53:47,208 ERROR [        ome.services.util.ServiceHandler] (erver-6458) Method interface ome.api.StatefulServiceInterface.close invocation took 96008

2019-07-05 08:53:47,209 ERROR [   ome.services.blitz.impl.RawFileStoreI] (erver-6458) Error on close, stage1

I’ve uploaded the full log to my OneDrive
During the past week or two, we’ve uploaded quite a lot of data ( 100+ files in CZI format, each file having 2-5 GB)

After a reboot, it seemed to work again.

Any idea what this coud be?