Steps:

1. Kill the player (/kill) that wants to use the command block (make sure he has op, and is in creative mode).

2. The player can no longer right click to access command blocks.

3. In order to access the command block again the player needs to reconnect to the server, once he will die again the bug will repeat itself.

I noticed this issue while building an adventure map.

Plugins that are on my server:
*Worldedit
*Voxelsniper