Just pure installation without any extensions or some other modifications? If db is ok and it is just so called fresh forum, without any additions.. you may try to replace current files with new ones (just in case download a new zip and compare hashes).. I would replace all the files (except config and maybe few folders, that are mentioned in updates/upgrade instructions). If you have modified something or enabled some extension/s, then redo everything and disable everything.
"When answer lies in the question,.. question becomes redundant!"