Hi Mhindley and welcome to GoY. We hard pruned are Clematis montana last autumn and it is growing away fine. Any chance of you adding a photo of the 'yellow foam' to your question please.

Sounds as if the plant's developed slime flux - the time to prune a montana clematis is actually late spring, immediately after its flowered. When you pruned it in autumn, some bacteria has entered the cut ends and infected the plant, and it's now producing this frothy stuff. Either that or damage to the woody parts enabled the infection. Late pruning is something you might get away with - but not necessarily. Best to remove the plant and its roots - once you've done that, it'll be fine to replant.