Problem Switching Template Themes

Problem Solved - I think!

I removed Modzzz "Points" module which I don't need anyway . Cleared cache's etc., and it now seems to be working properly.  

I also have no idea. Never seen the error before.

I've previously written about this, but the heading and my description weren't search friendly.

This is NOT an issue with Cheetah.

I can sometimes swap all the themes before logging in, but when I log in, I get a 500 error. I have to close the browser and start the site again.

I placed the site on a local server and all works as expected. I can change templates before logging in or after. I'm using Chrome, but FF is hit and miss. Every second choice may work, or get the 500 error.

This seems to indicate it's a server error because the local server is probably configured differently.

The server error message for the 500 error reads:

2024/04/29 23:34:54 [error] 2093184#2093184: *94418 upstream sent too big header while reading response header from upstream, client: 120.12.241.62, server: mysite.com, request: "GET /index.php?skin=evo HTTP/2.0", upstream: "https://134.203.97.374643/index.php?skin=evo", host: "mysite.com", referrer: "https://mysite.com/index.php"

The folk at Stack Overflow et-al have differing opinions and some say it's to do with the buffers being too small. Others have a different opinion.

Deano earlier suggested cleaning cookies, cache etc, but that doesn't help.

I'm lost.