opened it with electrum and everything seemed fine at first -snip-
Did Electrum showed you any error when you load the backup wallet file and typed its password?Because it should be able to detect if the extended (master) private key cannot derive the xpub at least one of the address in its list,
That usually results with the error that you've mentioned.
My reproduction result:
Restoring the seed should be sufficient in issues like that. (unless the wallet is forged)
But make sure to install the correct Electrum binary since a lot of fakes are showing in various search engines.
Only download it from: electrum.org/#download
And verify its signature: bitcoinelectrum.com/how-to-verify-your-electrum-download
-snip- and when i tried the private keys, for some of them i get a different address every time. meaning i import a private key into my wallet check address, then delete that wallet again and import the same key again, i suddenly get a different address then the time before.
Whatever wallet that is, do not use it.Inconsistent import shouldn't happen in any well-written wallet.