Serv-U is using the outdated version jquery ver 1.10.1. Kindly have this updated
One of the fixes listed against Serv-U v 5.1.7 is:
00039051, 00081081, 00195143, 00263829, 00260387, 00218061, 1362493, 1338156 Updated jquery libraries
If you have concerns about jquery versions it may be worth upgrading to see if this fix resoles your issue.