Hi Jake,
this can work, but I never tried that. I have separate repositories with separate ElasticSearch instances and mongodb instances for every branch I need. You can always go up (since we must support upgrades) but going down is a different problem.
So I suspect that there lies the problem. If you want to work with 2.5.1 either clean up (like elastic search, mongodb and build (mvn clean in graylog-projects-internal) or start a new from a fresh repro.
This is not much of a help, I know. But I guess you are right with your assumption that the problem lies with guice.
Greetz,
Konrad