Problems with plugin activation on the server.

The problem is this: The site is installed on the regular server (not free one). When I upload a new plugin, everything is ok. I use FireFTP. When I try to turn the plugin on in my Dashboard, Firefox shows only a blank, white page, and nothing more I can do in the Dashboard. Everything is blocked, and whatever I click on, the result I get is a white, blank page. The only solution is access via server tools to delete the included plugin and everything is back to normal. That is happening already with a several plugins. Note that site is all the time running and have visitors. What is the problem and what is the solution?
Thanks!