TLS error, but only on one directory

I am running Filezilla Server 0.9.41. No settings changes have been made but over the last few days users get this error when they try to enter a particular directory. All other directories work fine:

Error: GnuTLS error -9: A TLS packet with unexpected length was received.
Status: Server did not properly shut down TLS connection
Error: Transfer connection interrupted: ECONNABORTED - Connection aborted

I've tried renaming the directory, but this makes no difference.

Any ideas?
thanks