I cannot add products to VELUX ACTIVE from my wall switch (KLI31x) like I did before. What should I do?

When you add your VELUX products to VELUX ACTIVE or VELUX App Control for the first time, you exchange a pairing key between the gateway and the product. In case you decide to remove the gateway from your home (or make a factory reset), this key is deleted from the gateway but remains in the product(s). Attempting to add the product(s) to the gateway once again now fails, as the the old pairing is blocking the new pairing.
Unfortunately, the only way to proceed is a factory reset of your VELUX product(s). 
 
For details on how to factory reset a VELUX product, please refer to this video guide.