I am not sure what sort of error is that but when I click to edit any of my posts in Wordpress, I get a page of gibberish. I am using Wordpress 2.9.2 and there is no issues editing pages. I have tried clearing my browser cache and cookies as well.
I don't have a cache plugin but I have XML Sitemap generator that uses gzip compression? How do I go about it? It seems that only editing posts, not pages is being affected. Another thing is that I can do quick edit but not edit the posts.
You should turn off gzip compression manually using the settings page or deactivate this plugin and install another one (e.g. Google XML Sitemap).
I tried deactivating all my plugins but I still get the same results. What's more? I realised that while I can get to the 'add new' post page, I get gibberish when creating new posts or trying to thrash current posts.
This error might be caused by another type of compression, e.g. deflate. A page of gibberish usually shows if you try to read a binary file. I am not sure if this is your case, but I had a similar issue. When I removed compression, everything was OK.
In the end, the issue was resolved after overwriting Wordpress with a fresh download. It seems that the copy originally residing on the computer got corrupted.