@edrokov 

Uw  vraag in het Nederlands graag.

Hallo @edrokov ,

Do a reset: 

Thank you for the suggestion but it didn’t help.

Anyone knows how to order a new KPN Box 12 (or whatever modern version)?

Ok @edrokov 

Call with 0800-0402 for help. 

Thanks, they have concluded there is a hardware problem with optical modem, and somebody would come tomorrow for a check/replacement.

Ok, the final note: the technician has explained one specific KPN Box 12 batch is known for having faulty power adaptors, and my Box is from that batch. He has replaced the Box and power adaptor and everything works as it should for more than a day now.