DETAILED ANSWER · UPVC DOORS AND MECHANISMS
Is a sticking euro-cylinder different from a gearbox problem?
Understand common symptoms across cylinders, handles and multipoint locks.
Yes. A euro cylinder is the part the key enters. A gearbox or lock case operates the door's locking points.
A key that sticks can involve the cylinder, while a handle that fails to move the multipoint points may involve the mechanism; symptoms overlap, so inspection matters. Describe exactly what the key and handle do. A euro cylinder contains the key-operated part; a gearbox or lock case drives the bolts or hooks.
That difference helps describe symptoms, but it does not establish which component failed. Note whether the key turns, whether the handle moves and whether the locking points travel. Do not remove the cylinder to test it, especially if the door is shut or the property would be left unsecured.