Just guessing but this has happened.
On a few rare occasions.
The host upgraded the server and changed the owner/group that your account is allowed for.
Or changed the owner/group on some files and forgot to assign them to your account.
You could check it in ftp with like filezilla.
A site I am looking at now is 504 505
This is not the file permission setting which i would review at the same time
Since the forum runs I would check the ubbthreads.php owner group against several other files in other folders,
such as scripts,lib,includes..
If the owner/group is different across the files contact your host to make sure your account owns that group.
The only other thing I can think of is possibly a corrupted file or your server is dropping out during the post.
Server logs should show that.
It could just be possibly the server is overloaded at certain times and drops out. That is what happens to a popular site with shared hosting.
For that matter you could look at the database and check to see if the body and subject is populated with something.
But it would be nice to post the url in your profile or post so somebody could take a peek.
Last edited by Ruben; 07/26/2018 5:02 PM. Reason: added comments
