Weirdly enough, we are getting this error even if the transaction date is within the posting period. “The transaction date is not within the date range of the selected posting period. Click Go Back, then do one of the following: Enter a transaction date that falls within the date range of the selected posting period. Choose a posting period that includes”

Would investigate the accounting period record directly to make sure the start/end dates are what you expect.

Start and end dates are correct though

what transaction type? are there scripts or workflows on that type that might be changing the date or the accounting period?

No scripts and workflows. We just recreated and it worked fine