Server Not Starting.

I've been using Aternos severs for a few months. Never had this issue until I started using singular mods. Anytime I try to start my server with any custom map installed, I can barely ever get it started. Is it because I'm using a mod? Is it because the custom map is too big? Actually trying to film a series, and Aternos is was cheaper than buying my own server. The server it self with load in and start, but it won't ever give me the timer, and it shuts itself down. I've tried getting into the server while it still says "Starting..." but it tells me connection was refused. Please help!