I have a task running online in Inquisit, where participants complete the task multiple times on their own computers. One participant, who has completed the task twice already, just got this error her third time: "Inquisit was unable to contact the server to verify that this script is licensed. The script will not be run." 

She reported that she's using a Mac running a slightly older OS, 10.9.5. Is this likely the problem? It seems strange that it should have worked before but no longer. I saw previous posts with this error talking about enabling TLS, and she's done that in both Chrome and Firefox. Any suggestions?

TLS could be the problem. Enabling it in Chrome or Firefox may not help -- the browsers use their own TLS libraries, whereas Inquisit relies on the operating system's. In other words: TLS ought to be enabled at the OS level. OSX used to ship a fairly outdated OpenSSL library, which did not support more recent TLS versions. Doing a system update might help.