As you may know from your own experience, and incompatible theme or plugin can paralyze your entire website after an update. We try to solve obvious or known problems directly. Otherwise, we will restore the backup and contact you with a recommendation on how to proceed (e.g. which plug-in needs to be deactivated/replaced).
Fixing the problem is not covered in the maintenance service. We will give you a cost estimate and will be happy to fix the problem for you. You (or your web designer) can, of course, look for a suitable solution yourself.