Probably they are hitting a license limit. The software, like almost all licensed software, has always had a limit to the number of PCs on which the license key can be installed. It's listed here:
http://www.thetournamentdirector.net/faq.html#faq3, and in the
license.txt file installed with the software, and must be agreed to upon installation of the software. For the TD software, this limitation has always used "the honor system". While the vast majority of license holders do honor the license agreement (and it isn't hard - personal licenses are allowed to be installed on up to 3 PCs at a time), there is a small set that have abused it, and pretty severely. So unfortunately, in that regard, the honor system experiment failed and I had to enable server enforcement.
Email details (license key info and/or email address of person who ordered the license) to support@thetournamentdirector.net and I can look into it. It could be a number of things but I can't say until I lookup information on the particular license(s) with which you are having trouble.