Sorry, looks like I missed that in your original message. We've made a lot of changes related to database connections since then. If you can make a backup and upgrade to v1.5.3, I think that's likely to solve the problem.
My best guess is that you're running into the issue reported in this thread. You can read through the thread for some ideas to get useful logging. In particular, this thread may be helpful. Once you get 500s, it's safe to docker-compose stop && docker-compose up --detach to restart.