If you are trying to convert a .FLV file using OBS Studio's built-in remux-tool but you get the following error:
recording remuxed but file may be incomplete
OBS is telling you that the disk space on your target drive is full and because of that OBS can't continue to remux your video and stops the conversion. To solve this, clear some space on the target drive that you are exporting your video to or change the target location to a hard-drive with available disk-space on it.
There should be a pre-check implemented into OBS that checks that there is enough disk-space remaining on the drive before remuxing to help users get around this issue.
Hope this helps.