Help: Getting 500 error message when trying to access wp-admin

Hi -I hope someone here can help me as I have no clue what to do at this point.

I have changed the domain webspace root folder (/uuu) from the actual blog to a preview page (/preview) and now back to the original older (/uuu). Stupid move I guess. The result is that while the website still works perfectly fine as it did before the initial change to /preview I can no longer access wp-admin and only get a 500 error message when trying.

I already renamed the htaccess file without any impact.

Anyone any idea what to do?