Finally got to the bottom of the issue that user was having with directory names in FileSieve.
Turns out it was due to a Modifier plugin affecting the entire path of items rather than just the path after the root destination.
FileSieve creates directories within a destination the user specifies, but the Find Replace modifier plugin was performing its modifications on the entire path, including the drive letter. There's absolutely no point to this (it was an oversight, originally - now corrected) as the destination already exists and cannot be modified (eg. changing case).
This user was using the Find Replace modifier in a really good way - replacing invalid characters with valid ones so FileSieve doesn't skip out on them. But using the previous (incorrect)
entire-path-is-affected replacement style replaced the : colon character (as part of the drive letter) with a suitable replacement as specified by the user.
Oops. Now fixed; hopefully verified by the user himself as I've done a test build. A full release will be made shortly after verification.
While I was working with FileSieve anyway I also fixed some other stuff and re-instated the Invalid Filename Editor to replace any invalid characters encountered within filenames - a more consistent and low-level version of the Find Replace substitution method above.