Success!
Transfered funds from watch-only to standard
I did this (thanks to @o_e_l_e_o):
created a new standard electrum wallet with seed
after that i clicked "wallet" -> "sweep" -> pasted private-key
funds are now in tx from paper wallet to standard electrum wallet.
waiting for confirmations...

Thanks a lot, bitcointalk forum community rulez!

@LoyceV
Fortunately, it wasnt HD wallet but an old paperwallet address that was in memory(?) of electrum appImage. In all confusion and panic, I thought it is a HD wallet but it wasnt.
I thought I created this wallet yesterday because I have a lot of HD wallets that I dont use.
Anyway, I found the private-key on a piece of paper.