Check to make sure the settings inside OOTP match what the schedule file specifies. If there is any mismatch in the details, the file won't load (e.g. league size, alignment, balanced games yes/no, interleague yes/no, number of games in schedule).
Sometimes it's not clear if you've actually had the game regenerating a schedule from scratch. Just changing the schedule settings does not actually create a new schedule file.
I had an issue similar during testing where I was trying to load a schedule file and it wouldn't go through.
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