Just one last follow-up on this topic.
Finally, it worked, and I was able to transact those 30000 sats.
Conclusion: BlueWallet was working just fine.
I think that the assumption of @Rath_ was correct
If the receiver has more than one channel, your wallet might have found a path to a channel which had only outgoing capacity.
The receiver opened some more receiving-channels and my transaction went through.
Thank you all for your help!