How to troubleshoot general module issues? My site has a somewhat complex setup, so I therefore have a good number of modules installed. As a result, sometimes different things will randomly stop working, and I am not sure what exactly the cause was. Posting in issue queues can only go so far, and I'd like to start learning how to troubleshoot these problems myself. 

For instance, I'm currently using Entity Reference Prepopulate to help prepopulate fields in a "Review" entity type. I'm also using LoginToboggan to redirect users to a login page when they don't have permission to view the current field. Upon login, they are supposed to be redirected to the last page that they were on. So, with Prepopulate, the URL is something like http://www.gigindie.com/node/add/review?field_review_venue_reference=235, but after logging in with the LoginToboggan login form, the user is redirected back to http://www.gigindie.com/node/add/review instead of http://www.gigindie.com/node/add/review?field_review_venue_reference=235, which does not include the Prepopulate portion of the URL.

Obviously the above example is a specific use case, and help with that would be appreciated, but this question is a more general one. How do I go about figuring out why a module might not be working, and which other modules are conflicting with it, if any? Are there any tactics that would help specifically with troubleshooting module malfunctions?
My first port of call is usually the reports in /admin/reports, in particular 'recent log entries' (use the filters) and 'status report'. Also the PHP error logs on the server can be useful.

Also, keep an eye on Javascript errors by using your web browser's javascript console/log, as you attempt to do the broken activity.