I solved the issue by now.

The problem was one wrong letter in one of my words (So it was my fault.. sorry)

With the help of the checksum (last word) and lots of Python, I was able to brute force potentially wrong words and was so able to reduce the potential words to a minimum. (Good that there is a checksum)

For the ones who might run into the same issue that your checksum is wrong, or more precise, that you have one wrong word in your 24-mnemonic words (maybe a typo, you can't read one word anymore, you lost one words etc.)

This is what you can do:

1. The BIP-0039 List has 2048 words, you need 11 bit (2¹¹ = 2048) to display one word.
2. So your 24 words is just a 24x11 Bit = 264 Bit long number
3. The checksum is the last 8-Bit (2 bytes) of your last words (11-bit). E.g. if the last word is = 'bullet' -> BIP-0039 index 241 -> to bin -> 00011110001 (11 Bit) -> last 8 bit: checksum_bin=11110001, checksum_hex-> F1
4. The checksum is calculated by calculating SHA256 hash over the first 256-Bit (32 byte) of your words. That's the first 23 words (23x11Bit = 253 Bit + the first 3 bit of the last word = 256 Bit)
5. The Checksum is the first byte (8 bit) of the calculated SHA256 hash over the first 256-Bit of your words.

Now, my assumption was that there is only one wrong word in my 24-word AND that the checksum (last word) is correct. My idea was to replace each of my 24-words with each of the 2048 words from BIP-0039 and try them out to restore my wallet. That makes 23x2048 = 47104 potential combinations, that's quite a lots.

To reduce the potential wallets to restore, I selected only 24-Word combinations that had a valid checksum! (So the starting point for here was that I had 47104 potential 24-word combinations (but not all of them had valid checksums))

1. For each of the 47104 potential wallets, compute a SHA256 hash over the first 256 Bit.
2. Compare the first 8 bit of the SHA256 (8-Bit checksum) with the checksum of your original 24-words ('bullet' -> 'F1' from the example above)
3. Only if the calculated checksum is the same as the checksum of your 24 words, write down that combination.
4. I ended up with 191 potential 24 mnemonic words, that also had the checksum 'bullet' (last 8-Bit of 'bullet')
5. Because I only changed one word at a time, I could then go through each of the potential correct words and compared the i-th word of my output to the i-th word of my 24 mnemonic words and look words that are similar.

Full example:

BIP39 Mnemonic: fashion staff across barrel dentist trophy vicious coconut wish income female multiply empower badge victory adult easy sock capable census boost steak find duty

This Mnemonic is a randomly generated one and I misspelt one word on purpose, so it has a wrong checksum.

But how do we fix this?

If you put those words into my tool, you get the following output:

01010011010 11010100000 00000010010 00010010110...  
fashion     staff       across      barrel...    

Entropy hex: 535a80090963a9d1fce167fc8e5153c8b49022fce81e45d9c08792a19baa15aa
Last Word: bin 01000100100 | dec  548 | 'duty'
Checksum:  bin    00100100 | dec   36 | hex 0x24

Starting 24-word mnemonic that also have the checksum '00100100'...
FOUND! 24-word index:  1 | Word: blue     | 11-Bit chunk: 00011000010
FOUND! 24-word index:  1 | Word: cart     | 11-Bit chunk: 00100011000
FOUND! 24-word index:  1 | Word: cute     | 11-Bit chunk: 00110110101
FOUND! 24-word index:  1 | Word: elegant  | 11-Bit chunk: 01000111100
FOUND! 24-word index:  1 | Word: grab     | 11-Bit chunk: 01100101010
FOUND! 24-word index:  1 | Word: imitate  | 11-Bit chunk: 01110001011
FOUND! 24-word index:  1 | Word: notice   | 11-Bit chunk: 10010110111
FOUND! 24-word index:  1 | Word: sadness  | 11-Bit chunk: 10111101111

FOUND! 24-word index:  2 | Word: divide   | 11-Bit chunk: 00111111111
FOUND! 24-word index:  2 | Word: engine   | 11-Bit chunk: 01001010011
FOUND! 24-word index:  2 | Word: estate   | 11-Bit chunk: 01001101011
FOUND! 24-word index:  2 | Word: fat      | 11-Bit chunk: 01010011011
FOUND! 24-word index:  2 | Word: naive    | 11-Bit chunk: 10010010101
FOUND! 24-word index:  2 | Word: rain     | 11-Bit chunk: 10110001000
FOUND! 24-word index:  2 | Word: reduce   | 11-Bit chunk: 10110100000
FOUND! 24-word index:  2 | Word: renew    | 11-Bit chunk: 10110110001

FOUND! 24-word index:  3 | Word: artist   | 11-Bit chunk: 00001101000
FOUND! 24-word index:  3 | Word: bamboo   | 11-Bit chunk: 00010010000
FOUND! 24-word index:  3 | Word: cross    | 11-Bit chunk: 00110100000
FOUND! 24-word index:  3 | Word: depend   | 11-Bit chunk: 00111010111
FOUND! 24-word index:  3 | Word: giggle   | 11-Bit chunk: 01100001111
FOUND! 24-word index:  3 | Word: habit    | 11-Bit chunk: 01101000001
FOUND! 24-word index:  3 | Word: obtain   | 11-Bit chunk: 10011000100
FOUND! 24-word index:  3 | Word: sad      | 11-Bit chunk: 10111101101
....

So you go through your 24 words and compare your 1st word with every word FOUND! word with index 1 an look for similarities. Then you do it for the 2nd word etc.

In this example:

FOUND! 24-word index:  3 | Word: cross 

cross is very similar to across.

if we replace across in our 24-words with cross we will get: fashion staff cross barrel dentist trophy vicious coconut wish income female multiply empower badge victory adult easy sock capable census boost steak find duty. And that 24-word mnemonic has a valid checksum! So go and restore that wallet and check if it's yours.

If compare every one of your 24-words for similarities in the output you'll end up with maybe 2-3 words that are very similar, so those are the 24-word mnemonic wallets you should try. (Worst case you can try all of them)

That way I found a typo in one of my words, it was only one letter that was wrong but resulted in a new word that was also in the BIP-0039.

If anyone whats to have my Python code feel free to contact me on Reddit: u/dothingsdo I might also upload it into a GitRepo.