Clematis weeping sap .

Afternoon all some advice greatly appreciated here , I have (or had) a well established Montana clematis on a South West fence , just as the growing season started I noticed  the lack of growth and foliage ? There was some buds  that did flower but now the foliage has all died off and I have cut it right back to a foot or so of ground level however I notice the woody stems that are left are producing what looks like sap and are spongy  and the stems now seem to be rotting?  I'm assuming the worst and this favourite clematis has bit the dust but what could the problem be ? I stay in morayshire North East Scotland.  Thanks in advance.