I'd say that your middle chainring is probably quite worn, to the point where the teeth refused to let go of the bottom run of the chain and carried it up and around until it jammed under the derailleur and top run of chain. As to how to avoid it, replace chainrings before they get to that point of wear. I can't see that using an 11-speed chain on older chainrings can cause a problem, as the width of the rollers has not changed as the number of speeds has increased (at least up to 11-speed). Leonard Zinn explains it here: https://www.velonews.com/gear/tech-faq-chain-width-explained-compatibility-queries-answered/ .
I have unused chainrings dating all the way back to 6- and 7-speed days, the width of the teeth on them is no different to that on modern chainrings, and I've run 11-speed chains on the older rings without any problem. 13-speed Campagnolo Ekar is the only groupset I have where the internal chain width and chainring tooth width have both been decreased from the widths of fewer speed setups. Nick -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/rbw-owners-bunch/4aa7f0fc-0774-45aa-bc95-ab5f92f76b5bn%40googlegroups.com.
