As you only have ONE derailleur, it is difficult to determine what exactly your problem is.
Also, the term "Works", has WILDLY different meanings for different people.
I could make some educated guesses - If this is a crank-drive, and the chain slips off in anything other than one particular gear, then this is a chainline problem which is normally easily fixable, however most bike shops will not touch ebikes, and this may be unique to your particular motor mount. You would need a ebike shop, familiar with crank drives, in Belgium. Got no help for you there.
Might need a chain tensioner. Might just need a stop limiter adjustment. Might need motor re-mounted. Might need a new ebike.
DEFINITELY need to be able to explain the problem in more detail to get anything remotely approaching a usable answer.