oh, this is problem is hardly to reproduce;(
I can easily reproduce the problem on my computer. I just open Outlook Experss, click "Create Mail" and try to insert various texts using Flashpaste (just click Ctrl+I, choose test using clipboard and press Enter). In some cases text isn't pasted. Maybe it depends on the system configuration or another software installed but I don't know what could affect Flashpaste.
Try following, open option dialog and uncheck the "Restore Clipboard content after pasting a Clip" checkbox. Maybe this can help.
Unfortunately it doesn't help.
What about pasting via Flashpaste Tray Menu?
It seems to work correctly.