Sometimes manual import fails with a "Unable to load files for manual import. Check logs for more details." error.
In the Sonarr log you may find a "Request Failed. GET /api/manualimport: Sequence contains more than one element" message, indicating that at least 2 shows have the same "cleantitle" (name).
Open the database manually, for example with SQLitebrowser, and execute the following SQL statement:
GROUP_CONCAT(Id, ',') AS Ids,
GROUP_CONCAT(TvdbId, ',') AS TVDBIds,
GROUP BY CleanTitle
HAVING COUNT(*) > 1
If you get rows back, then this means that you have shows with duplicate (not unique) clean titles.
Unfortunately, Sonarr uses a technique to determine this unique title, which results in "Flashforward" and "Flash Forward" having the SAME clean title. At least now you know which shows are causing the problem and you can now spend time cleaning up those shows.