Thanks, everyone, for your help! I had to test several configurations, including with and without scanning the QR code and rebooting the OS. In the end, it turned out to be a missing or incorrect prefix.
Now for the "No inputs found" message, click the "Info" button on the upper-right hand side of that 'Sweep private keys' window.
If unsure or if the script type isn't indicated in your paper wallet, you may type your private key with three different script types.
You can skip "p2pkh:" and just type the WIF private key for it to also include P2PK UTXOs, then include the other two.
Thanks to this message, it became clear that the problem was probably with the address that was entered incorrectly.
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An additional uncertainty was that I had entered the wrong public key into blockchain.info and received no clear error message, so for a while I thought I had previously sent all my BTC balance to a wrong address, for example, the one of my one paper wallet.
What type of script (address) are your funds allocated to?
When scanning private keys in Electrum, you must specify to it which type of script Electrum will scan for the amount, which are:
p2pkh: Legacy address that always starts with
1p2wpkh-p2sh: Nested segwit address, starts with
3p2wpkh: Native segwit address, starts with
bc1
Depending on the address type, you must include the above options as a prefix followed by the private key, e.g.:
p2wpkh:LorKxyz(address...)
Note: for legacy addresses, just enter the private key without the prefix, using the prefix in private keys/legacy addresses is optional.
p2wpkh was the solution

Thank's again for your great help!