Problems when upgrading WordPress?

My recent upgrade to WordPress 3.1.4 went smoothly. However, the following morning on trying to view my site I received the following error message:

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error was encountered while trying to use an Error Document to handle the request.

This seems to occur everytime I update the site

My hosting company informed me that the permissions for my admin folder were incorrect and corrected them. This got me into the site, however I gained two thick horizontal seemingly neverending white lines along the bottom of the theme, that meant I could scroll horizontally forever more! Additonally, a 404 error was received for the admin login page. I resolved that issue by changing the permissions via Filezilla, but the odd design glitches persisted.

I assume that the initial problem was caused by upgrading WordPress. Because of this, or perhaps because of the ‘fix’ in changing permissions, it scrambled the theme design? However, I am unclear why the permissions suddenly changed just because I upgraded to the latest WordPress version.

I reinstalled the theme but the problem persisted. Finally I gave up and reinstalled WordPress totally and saved my theme settings etc to re-upload.

Has anyone else experienced theme problems after upgrading WordPress? Clearly, if this is a host set-up problem as I suspect it needs to be resolved else I need to host my site elsewhere – I can’t keep re-uploading my site every few weeks!

Theme, wordpress or host?

Be glad of the thoughts of the more experienced!

Alex