And to confuse things even more, when I add the same version of the project itself to my Gradle build instead of depending on the jar, everything builds fine.

These kinds of errors can mean that you have version mismatches in your dependency tree.

Yeah. We’ve tried to unify all the versions of things, but that doesn’t seem to help.

Gradle has a bunch of command-line options that will print out the tree and any overrides, you can also query a particular jar to see its dependency list. It may help you diagnose the problem. Look for common transitives with an overridden version. Less likely but possible problems: JAVA_HOME or CLASSPATH being overridden, dynamically loaded jars, corrupted POMs, or java module problems if you are using 9 or above.