There's been some good advice on fixing the issue.
But I'll add...
Jockey box lines should be cleaned and sanitized after every use, not just at the end of a season. My process, on same day as event:
- push beer out
- CIP the Beer Lines/Plates/Coils/Faucets with recirculating BLC
- Rinse
- Sanitize with properly diluted Iodophor (do not stain the lines with Iodophor solution that is too concentrated.
- Blow CO2 (or Air) through Lines/Plates/Coils/Faucets to dry
- Leave Faucets Open
- Remove, disassemble, clean, let dry, and reassemble Liquid QDs (don't reinstall during storage)
- Store Box with Lid and Faucets open (to continue drying)
I've seen some pretty nasty stuff in (and on) club jockey box lines and in the boxes themselves. And once there's something growing, it can be pretty hard to get rid of it permanently. For whatever reason, a lot of people think that jockey boxes don't need the same level of care as other cold side equipment.