If there's webbing amongst the leaves, my money's on box tree caterpillar, although its odd you've not actually noticed the caterpillars themselves yet. Of all the problems Box gets (and its an ever increasing list) the only one that goes with webbing is the caterpillar. The caterpillar is the larval stage of a moth and was first found in mainland Europe in 2007, so it's been around for a while, not quite so long in the UK as over there. Eggs are often present under the leaves if the caterpillars are not hatched yet, so check beneath.
This link https://www.rhs.org.uk/advice/profile?PID=88 lists various problems from which Box suffers - you can click on each one for more information to see for yourself what you think.If it is Box caterpillar, picking them off by hand is useful, but spraying may be required. Note the spray has to be pretty forceful in order to penetrate the webbing. Pesticide products recommended in the link may not be available under the same names where you are, but there should be products using the same active ingredients.