I found an usb stick with a wallet.dat.
There is also an old notebook with Bitcoin Wallet v.8.6-beta on it. Unfortunately I can't sync it on my old notebook and therefore I am not able to send BTC to another wallet.
I then installed Bitcoin Core on my new PC and put the old wallet.dat in the directory. The bitcoin showed up in the Bitcoin Core Wallet on my new PC; BUT when I tried to send them it says "passphrase is incorrect".
I then doublechecked my passphrase using "change passphrase"-feature on the Bitcoin Wallet on the old notebook and it worked, meaning the passphrase must be correct! I could even do a transaction and put the passphrase (BUT as you know, because it is not synced - the network will not accept the transaction...)
In other forums they recommended using Electrum instead to import wallet.dat. This did not work for me.... I am not expirenced with bitcoin and IT in general....
So, do you guys have an idea what causes the incorrect passphrase in Bitcoin Core? Or could you recommend a trustworthy company to solve the problem?