Unable to find JQL function ‘inSprint’

In our JIRA deployment we use several JQL functions from the excellent Script Runner plugin.┬áBut suddenly the ‘inSprint’ function stopped working, and when trying to use it you get the aforementioned error.

Turns out that the plugin has a certain bug which causes some of their functions to become unregistered in JIRA.

The solution: Disable and re-enable the plugin. Works like a charm!