Hello viking83,

Since you are unable to change the theme in the Dashboard, try changing it manually:

• Within the folder where you installed WordPress (either public_html or a subfolder) navigate to the wp-content/themes folder and rename your current theme to manually disable it. This will force WordPress to switch to the default theme. Just keep track of what the original name of the folder was so you can easily change if needed.

This could also be caused by permalinks:

• Change your current Permalink structure: Change your permalink setting to anything else and save the new structure. Switch back to your previous Permalink structure and save again.

As previously recommended, replace your Wordpress core files. The point of this, is replacing/uploading the newly downloaded versions (this way you know they have not been modified/corrupted):

• Download the latest version of Wordpress, then upload & replace all files & folders - except the wp-content folder

Most of these steps are from our Troubleshooting Wordpress guide.

If your problems persist, it may be better to Contact Live Support, so you can provide account specific information, and they can replicate the error and investigate further.

If you have any further questions, feel free to post them below.
Thank you,

-John-Paul