Why is it that when a player is opped on my minecraft server, no one else can build/break blocks?
If there is even a letter in the ops text file nobody including myself can build or break anything. Even as owner rank if I were to op the letter 'c' I still would have this problem. I use Group Manager so it is most likely something with this.
Probably you are inside the spawn protection radius and that is why you can't break blocks. When there are no ops on the server, spawn protection is disabled, that is why you can break blocks when there are no ops defined.
For more details, see Spawn protection and server.properties (search for 'spawn-protection')