Today FlashDevelop got stuck on startup into an infinite loop of the following error:

Binary stream '0' does not contain a valid BinaryHeader. Possible causes are invalid stream or object version change between serialization and deserialization.

I found this really old forum post that said to delete the whole FlashDevelop app data folder.

So, I deleted/renamed the folder located at C:/Users/YOURUSERNAME/AppData/Local/FlashDevelop, and it works again!