Here's one that I hadn't come across before. It's my own fault for not paying attention, but I thought I'd put it out there in case others came across it. Not sure if this is expected behavior or not. Guessing that it is.
Using SQLExplorer locally on my laptop. It is connecting to a couple of files that are on my development server. One older data source from a previous project is via the direct internal network IP address of the server (192.168.xxx.xxx). The second is via the domain name of the server - an external network connection.
My single copy of FileMaker Pro 15 Advanced triggers the error saying that there are duplicate serial numbers in play and I get kicked out.
I'm assuming this is expected behavior as there's both an internal and external network in play.
Once I changed the data sources to be via the same network address all was well again. Also, remember to clean out SQLExplorer data sources when you no longer need them.