Require customers to email support, require a log file, have your log files show if mods are installed, auto reply that the customer should reinstall the game without mods and see if the issue persists.
If you want to get really snarky, figure out who wrote the mods and cc them on the reply saying "For your convenience we have included the authors of the installed mods on this email so you can work with them to resolve your issue."
You could even automate that entire process.
Require customers to email support, require a log file, have your log files show if mods are installed, auto reply that the customer should reinstall the game without mods and see if the issue persists.
If you want to get really snarky, figure out who wrote the mods and cc them on the reply saying "For your convenience we have included the authors of the installed mods on this email so you can work with them to resolve your issue."