WP do discard my edited text when loading a featured image

Several times, I have lost a lot of text while editing a new post. First it dissapeared since a plugin automaticly logged me out after 15 min (It was of course mre than 15 minutes since I pushed save, but it should not happen anyway since I was typing all the time)

I disabled the plugin (Secure login plugin)

Decided to give it a try again. I hate the feeling that I should do all typing in eg. word and then past it into the editor.

Today, I did a lot of typing and then loaded a featured image. When coming back to the editor, I saw that all my last typing were gone again. After som cursing I decided to contact you again to see what this is, and how to solve it?

This hasplanted a deep mistrust for WP inside me, after using WP for years. I hope this can be fixed.

/ Lars