Just a guess…
Is your Project Settings/Decompression Fallback ticked?
Else you have to have your server set up correctly, to broadcast the compression type in its header for the browser (this is new in 2020.1).
I’ve been dealing with buttload of similar errors these days, and it’s behaving somewhat reasonably now (webgl builds loading on different platforms). The other thing is, the 90% ‘done’ - i always experienced it with ‘Developer Tools’ (firefox) open, but that was an ‘out of memory’ error. Same error with Chrome and it’s dev tools open (closing dev tools while the site loads [or reloads] solves this issue in that case).
Nothing else i can think of right now, but will come back if i remember something. I’d love to help, seeing as i myself have created like 10 threads in different places in the past 3 days, asking for help and not a single answer in either. Damn, don’t want anyone else to suffer as much, ain’t pretty :).
Also, 2020.1 has changed the WebGL output , the whole loader was updated - so you might wanna check it out (or check out the previous version, which ever you are not using), if that solves your problem.