This could be due to two possible causes.
One is there was a bug that presented this behavior but was squashed in newer releases.
Or it could be your php settings are incorrect such as max_input_variables.
Try the link below to download the ubb test script to check your hosting settings for compatibility.with UBB

https://www.ubbwiki.com/latest/system-requirements-test
If the max_input_variables are suspect read up at:
https://www.ubbwiki.com/article/view/25/issues-with-the-php-max_input_vars-setting.html

Remember the UBBWIKI is your friend.

PS
I would also suspect your host has upgraded you to php7.x which your UBB version is not rated for.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers