setback, mishap, bad luck
[noun]
[de te-gen-slag, de te-gen-sla-gen]

A "tegenslag" is generally more serious than a "tegenvaller" (something turning out in your disadvantage unexpectedly). It is composed of the words "tegen" (against) and "slag" (blow, stroke).

A common usage is "Met tegenslag(en) te kampen hebben", see below.

Examples:
– "Marc en Sander hebben beiden met tegenslagen te kampen, vandaar dat er onlangs weinig DWOTDs zijn gepubliceerd." 
("Both Sander and Marc suffer misfortune at the moment, that's why not many DWOTDs have been published recently." Note that the verb "kampen" originally means "to struggle/fight" but is not used that way anymore.)

– "Het project heeft vertraging opgelopen omdat we tegenslagen hebben ondervonden bij de aanbesteding." 
("The project is delayed because there were some mishaps during the call for tenders.")

– "Na herhaalde tegenslagen heeft hij de hoop opgegeven ooit nog ware liefde te ervaren." 
("After repeated setbacks he gave up hope that he would ever experience true love.")

– "Hoe gaat het met de bouw van jullie huis?" – "De ene na de andere tegenslag, echt waar, ik wou dat we er nooit aan begonnen waren!" 
("How's the construction of your new house going?" – "One setback after the other, honestly, I wish we had never started it!")

Related words:
– Tegenvaller: disappointment [noun] [de tegenvaller, de tegenvallers].
– Pech: bad/tough luck [noun] [de pech, <no plural>].