In my opinion, everything was correct, so I uninstalled the wallet on the old laptop.
Correct means you also had the money in, right?
Some months later I wanted to check my wallet, and then there was zero credit (with an exit equal to the original amount).
If you mean a transaction, then I think that somebody has stolen your money. Sorry.
1. The old laptop, although uninstalled, may still have the original wallet, but if my assumption is correct, that will also be empty.
2. You didn't say anything about where you downloaded electrum from, whether you verified it and what version it was. You didn't say anything about the update you've made - was it a popup dialog offering a download link and you downloaded from there "Electrum 4.0" (a phishing clone of Electrum)?
3. If you always had
verified Electrum and indeed you had the money there at least once on the new laptop, you'll have to think on what software (malware) you've installed, what shady links you've clicked, where you saved the backup of the seed (mail, dropbox and other places you should not trust?) or who else had access to the laptop.