Maybe it is a good idea, that dumpwallet does not overwrite / purge an existing file.
I did not read the info of dumpwallet correctly and did a: "dumpwallet wallet.dat".
I assumed I had to provide the walletfile I want to dump.
I know, this was fully my mistake and it is no problem since I still test in non-productive mode...
But the wallet.dat was immediately purged, the node crashed and of course the wallet was lost.
Would be much better when dumpwallet gives back a: "File already exists!"