I've tried to send to my own, new ones and external addresses and they all do the same.
The wallet has a bunch of addresses on it, including the one which got some solo mining blocks but I'm not sure which one that was since. In the transaction history it just shows the incoming mined btc but not the address.
If you really do have solo mined blocks, you're looking at enough money to contact a specialist if needed. Someone like Dave, but be very careful who you trust.
I have found a way to see if a wallet.dat file is a Bitcoin file or an altcoin file.
Assuming you had funds there or generated any address, if you open it with Wordpad or any other text editor, you can ctrl+f and search "name"", and you'll see the list of addresses you had on that wallet, so if they begin with "1..." it will be a Bitcoin wallet, assuming it's not BCash or any other alt which uses also "1..." format but most altcoins use their own number or letter.