Ciao
It happened to me in the past
First, you should check if you can log in via SSH, if OPT is full you should not be able so you have to restart the SSH service on each ESXi host.
Then you connect via SSH to one of the hosts and run the command VDF -h to see the space occupied.
Then you can run the following command:
find / opt -size + 1024k -exec du -h {} \; | less
which lists you the files with a size + larger than 1024k
from this list, you can understand what is filling your OPT and decide whether to delete the file or not
It happened to me in the past that it was the raid controller logs (MegaSAS.log)