Yesterday I made some more tests on this issue and I made two changes:
First I added the user “NETWORK SERVICE” to the security settings of the bin folder (C:\Program Files\MongoDB\Server\7.0\bin) with read and write permissios. So the service can write a mini dump to the folder on the next crash.
Second I added the data (C:\Program Files\MongoDB\Server\7.0\data) and the log (C:\Program Files\MongoDB\Server\7.0\log) folders to the exceptions of the virus scanner.
Since then the service runs stable again. Maybe an automatic update of the virus scanner caused the issue because the service ran without any problems before.