I've had this error before.
For starters, check your plugin list. Check for updates. Even if there are misplaced plugins that shouldn't logically affect anything else, put them in the exact order that the developer has listed on their site. It's good for organization anyways.
After this, turn all your plugins off and troubleshoot, slowly turning them back on one at a time after testing until you find the erroneous plugin. Now that you have it narrowed down, figure out what it's doing that clashes with the Gallery plugin.
If you have a backup saved before the plugin in question is used, load it up and start from there, making your way back to where you were before. I've has enigmatic situations where I had no idea what went wrong and did everything seemingly identical from last time and yet saw the issue completely fixed just by reloading a backup from a couple days before.
If all else fails and you can't figure anything out, reconsider the value of the plugin. Maybe it isn't worth the trouble. Sometimes you can find clever ways to accomplish something similar. If you absolutely must have this plugin and you can't figure out how to fix it, contact the publisher and be as specific as possible.
