What does the message mean?
How to avoid it?

The message "CFTH07E TFIL task Synchronization error  <PART=PXFBGWS CR=-1 CS=00000B76>" means the CFTTFIL process runs into a Synchronization issue (most probably a timeout) with other process.

The users should check "WAITTASK" parameter (Inactivity time, in minutes, of the CFTTFIL task) in CFTPARM object. CFTTFIL will stops if this timer is reached and log the error message. For example, if WAITTASK=1, CFTTFIL will exit after outputting the message when there is no activity for 1 minute.

To avoid this, the WAITTASK value should be big enough.