can’t delete posts; post editing gives blank page, but works

My site gives a number of errors which show up in the search results here, but none of the solutions I’ve tried so far have worked. I suspect these problems are interrelated. Any suggestions will be appreciated. I am running 2.0.5

– I (the site admin) can not delete posts. At first the “Are you sure dialog showed up. Clicking OK to delete used to do nothing. Then I cleared my cookies and cache, and now it takes me to a blank page. If I try refreshing, I am given a warning about “POSTDATA”. I am using Firefox. In any case, reloading the site shows that the post has not been deleted. However, I can delete comments and links.

– When I edit pages, if I click on “Save”, the page is replaced by a blank page. Reloading the page shows that the changes have, however, been made.

I have the wordpress-tuneup plugin activated. If I deactivate it, I sometimes get errors called “Internal Server Error”.

Furthermore, my admin bar plugin’s “Edit this post” function does not work, since the post number is missing from the URL it generates. This made me suspect my permalinks, and, indeed my htaccess file has none of the 100’s of rewrite rules it used to have in it. I changed permissions and updated the permalink structure, but the htaccess file remains the same (it does have a WordPress-generated section in it, just not with the many posts’ rewrite rules included).

If you have any questions, please ask!