Unable to Enable/Disable plugin

Hi,

I am new to Elgg, recently i installed elgg in my computer (Ubuntu OS) to give it a try, it working great...however, now I facing a problem. I don't know why suddenly I can't Enable/Disable any plugin.

I got this error everytime I try to enable/disable a plugin.

Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 20 bytes) in /var/www/elgg/engine/lib/database.php on line 270

I thought it is a memory problem, so I try to increase the memory_limit to 256MB (innitially it was 128MB). But the problem is not fixed.

So I thought it was the plugin conflict. I remove all the plugin in the mod folder, and adding the plugin one by one to see which plugin that causing this problem. But whetever plugin that I put into mod folder, I still unable to disable/enable them. Seem is not the plugin fault as well.

I got no other clue now, any suggestion?

Thanks