1. Add-blog-post-tag-error.
2.add-photo-error-at-tag.
3. add-vide-tag-error
Check whether
{text key='base+tags_input_field_invitation'} is not missing language value in admin panel>>settings>>language.
Should be like this:
4. admin panel error message.
5. errors when commenting video, photo.
Check whether this tag is not missing value as well.
6. Main-page-editing-error.
Which widget did you try to edit?
Try to reproduce the error once more. Once you get it, check error log on your server, latest one should be related to this 500 error.
Also enable DEBUG mode just in case and try to set the widget again. Let me know the results.