My first inclination is you have some type of permission error.
You should never have to edit the database for normal permissions set in the control panel.
Now when I say permissions, it is not just file/folder permissions.
But other issues such as Suhosin or mod security which limits the maximum amount of data submitted or what is submitted,Could be a culprit.
As stated if you modified the code or are mucking with the database tables. Then who knows what the issue is.
Also it has always been frowned upon to delete forums, but to disable them and recycle them.
Last issue is what do you expect for a shared hosting plan?
Sorry for that last remark!
My 2 cents.
Make a full backup including the database.
Reinstall a virgin v7.5.8 in a test folder and link it back to the database in config.inc.php after you test it with a clean net new database.
Then link it to your old database.
See if that resolves your issue.
