As reported by laborix on the forum:

With Flatpress 1.2.1 the following PHP warning can occur when uploading files:
PHP Warning:
Undefined array key "uploaded_files" in .../fp-content/cache/e93fccb09cf8b04111b9595da102f3f4^%%9D^9D6^9D6FBCCD%%default.php on line 21
This error occurs when non-permitted files (Flatpress Uploader Blacklist) or incorrect Mime_Types of files are detected when uploading to Flatpress. There are two possible solutions to eliminate this uploader warning.

Needs to be inspected and fixed - thanks for reporting! :)