Everything is not as easy as we thought!
Yes, the engine failure haunts us still. After replacing the spark plugs, a test flight for verification necessary parameters and the appropriate documentation shall be executed according to LBA specification.
The same applies for other flights to ensure the absolutely trouble-free use of the new spark plugs.
This first flight took place yesterday; after issuance of the relevant documents and approval by the LBA, further flights are possible.
An observed pressure drop in oil pressure during the flight could be resolved directly by cleaning the oil pressure control valve.
Here's to watch again, but that it is an "old" aircraft with old components that must be handled and required by good experienced persons.