It will most likely be related to the “odd” characters, but probably only one. Somewhere there will be a forbidden (i.e. otherwise normally used as a “special character”) in code.
The best solution is to use Raw GEDCOM edit as you have tried, but in a slow, methodical way.
1 – Remove it all except perhaps a couple of innocuous words at the start. Save.
2 – Repeat adding a sentence or two at a time until the error re-appears, then back track and repeat using that last block of text but in smaller pieces.
3 – Continue to reduce the size of the block of text until it becomes clear which particular character(s) are causing the issue and remove them or replace with something better.
If finding the exact cause is not that important to you, the alternative is to transfer the text, which is anyway rather long for a note, into a PDF file and upload that instead.