I was having that problem. I was able to work around as follows:
In Unity (version 2020.1.3) I went to Project Settings -> Player and under Settings for WebGL, the Publishing Settings, and set Compression Format to Disabled.
It appears what is happening is a javascript file something.js is being compressed to something.js.gz, which is normally fine, but when it is uncompressed, the uncompressed version is still something.js.gz, and is thus not recognized as a javascript file by the web server. Seems a bug in the latest Unity version. Disabling compression gets around this for now.
I figured out the problem in Firefox by selecting Tools -> Web Console and looking at the error messages, and it told me which file it didn’t like, and I examined it with 7-zip.