Thank you for everyone's assistance and time.

I finally cracked the riddle!

I logged in to blockchain.com and used the walletID (the "05bf8b8a-639c-XXXX-XXX-87718ba56d23" part of the url) and an old "standard password" I used back then.
The site then sent an authorization code to the email address linked with the walletID. It's here that I got lost, because no confirmation mail was sent to any of my email addresses.
Then I remembered an email address I used way back then. Opened the mailbox and the authorization link from blockchain.com was waiting for me.
Click to confirm. Login. Move coins out! Yay!

So in short the wallet.dat file that accompanied the wallet address was the red herring!

Thanks again for the patience and help! (I must give kudos to nc50lc - it was your previous post that nudged me towards the finish-line.  )

(On a side note, if anyone is aware of a time machine or a way I can tell 2012-me to keep mining, please let me know. I'll pay handsomely.)

Thank you for your VERY informative reply!! I appreciate your time.

Seeing that the address doesn't seem to be useful in recovering the funds, this is what the transaction looks like:
https://www.blockchain.com/btc/address/12E34isxWUmH52oaePQsAJ9tBNZtbTZ25X
(It's pretty much Greek to me.)

I've been fine combing the old HDD in search of some more clues.
I found that I may have used Bitminter to mine. Could it be that the wallet is/was registered with them, or rather that my coins are with them. And seeing that they are dead, there's now way to recover the coins?

I know the comments/questions sound pretty stupid - but that's because they are...

As far as I can remember there wasn't a "12 word recovery phrase/mnemonic" back then. If I had to create one, I would like to believe that I would have written it down.

I don't know. It's quite a challenge to remember something you did 10 years ago when you cluelessly messed about with something.  
The fact that I wrote down a "blockchain.info/wallet/" leads me to believe that I could have one (the specific one) on blockchain.com. (Which is the address that responds with "The page you were looking for doesn’t exist".)

That was my guess. I created a login on the site, but whether I'm logged in or not, whenever I try the address it responds with "The page you were looking for doesn’t exist".
Is there any relation between the URL (looks like Hex?) and the actual Wallet public/private key? I.e. would it be possible to figure out the address of the wallet with the address being derived from the URL?
Could it be that they "deactivate" wallets when it's not been used for a long time, and then a way to reactivate it again?
I still have the email address(es) I would have registered with...

I did that which resulted in it opening a wallet with a zero balance, which supports my assumption that there is no relation between the address that shows a balance and the .dat wallet.


I understand that. Can I assume that the "12E34isxWUmH52oaePQsAJ9tBNZtbTXXXX" address is a public address and it should have a (different) private address for the public address somewhere?

I copy and pasted each and every "sec" value from the recovered wallet into a new file. There are 101 values. I created a new wallet in Electrum and pasted all the sec/private keys using the Import Address/Private keys option. (I don't know if it actually accepted all 101 values?) and the result was 0 BTC in the wallet. Which makes me wonder if the wallet.dat file has anything to do with the (12E34isxWUmH52oaePQsAJ9tBNZtbTXXXX) that shows a value.


Yes, with the latest version of Bitcoin core. However I don't see an option to Import an old wallet?
For what it's worth, I also found in the Downloads folder "bitcoin-qt.exe" (0.6.3-win32) which I'm guessing is the software I used back then. When I run that (with the old wallet.dat file in the correct location (AppData\Roaming\Bitcoin) the wallet also appears to be empty. (There is a notification that reads "URGENT: Alert key compromised. Upgrade required.) Which still makes me think that the wallet is unrelated to the address with a balance...

Could you please use smaller words - I have no idea what that means... 

I'll give it a try, thanks.

As many others I discovered an old Wallet.dat file on an old HDD. I've been through all off the other similar topics, but none was helpful in my case.
I also found an accompanying text file - which I usually do when I make notes.

Back in 2012 I installed all the necessary Bitcoin bits to see what the whole "Bitcoin" thing was all about. I did a bit of mining, and lost interest after a day or so. (As most people I regret my decisions back then...)

But anyway. Today I'm pretty much as clueless as I was back then.
In the text file there is two lines:
(I've changed some characters in case I might be unbeknownst giving access to my wallet.)
 
https://blockchain.info/wallet/05bf8b8a-639c-XXXX-XXX-87718ba56d23
When I visit this address I get a 404 page not found.

Then there is a value which I gather is a public address?
12E34isxWUmH52oaePQsAJ9tBNZtbTXXXX

I also have a wallet.dat file. I fed the wallet.dat file through the Python pywallet, and it recovered the content with 101 private keys. However all of these appears to be empty, i.e. contains 0 Bitcoins.
The above address appears only once in the recovered wallet and it looks like this:

 
...

"minversion": 60000,
    "mkey": {},
    "names": {
        "12E34isxWUmH52oaePQsAJ9tBNZtbTXXXX": "MyWallet",
        "1PnaH4bB44z6nVSvQVpoTekUriDTeWXXXX": ""
    },
    "pool": [
        {
            "addr": "16sGKPDPDukFqZdHpUGQhHwTtXNPEjXXXX",
            "n": 2,
            "nTime": 1342705737,
            "nVersion": 60300,
            "public_key_hex": "0296edee31e38d2e8e2a6467183e4cc1d3762cff34596012f2f3956d71e869XXXX"
        },
...



When I enter the address (12E34isxWUmH52oaePQsAJ9tBNZtbTXXXX) into electrum-4.3.2 it shows a balance of 5 mBTC. While it doesn't make me a millionaire (if only I knew back then...) it is worth getting into the wallet.
The wallet is however "Watch only"?

If I recall correctly there didn't exist a 12 word seed phrase back then.

So, will any of this get me into my wallet? Thanks in advance!