Bizarre 500 internal server error on post

I’ll preface this by saying that the symptoms are quite bizarre, which has made trying to diagnose the problem incredibly frustrating.

I recently upgraded from 2.7 to 2.7.1 and also changed themes to Vigilance. Shortly thereafter I began getting a 500 internal server error after publishing a post. I tried going back to 2.7, but the problem persisted. However, if I write a short test post for debug purposes it publishes with no problem. Even after repeated updates where I change the category, add tags, and other things, it continues to update as expected. But then when I write a more lengthy post I get the 500 error on publish. The post still gets written to the database and shows up on my front page.

However, a couple of plugins I use for posting (Twitter Tools and Event Calendar) don’t work when I get a 500 error. The post gets published as written, but the event doesn’t show up and Twitter isn’t notified. I checked the DB and saw that the event does not get written there when a 500 error is encountered.

I use 1and1 hosting and do not have access to error logs. I tried using a php error logging script they provide, but have not seen anything suspicious there.

One other behavior that induced the 500 error was activating the “Role Scoper” plugin. When I activate this plugin I get a 500 error, but this one wipes out the entire site; every page returns a 500 error. I have to physically change the name of the name of the plugin’s directory so that WordPress deactivates it just so visitors can access the site.

I’ve tried deleting and recreating the htaccess file. I’ve tried modifying it based on other support threads. No dice.

Any advice would be GREATLY appreciated since this is an incredibly frustrating error and is delaying the roll out of expanding my contributor roster.