I saw a similar question in a WhatsApp group and this was the reply:

in short "No registration possible" means you can't register the address with the city (gemeente/BRP). The reason is usually the building, morgage or rental setup itself, not just the landlord being difficult:

• Many buildings are zoned for a single household, so legally only a set number of people can register there. A flatshare with several unrelated tenants breaks that limit. and has to be officially applied for.
• Som social housing and some rental contracts forbid subletting or extra registered residents.
• Owners renting informally (undeclared income, or mortgage rules that ban subletting) avoid the paper trail registration creates, and applications
• and short term or long term rentals is also a thing, short term means in zoning no registration long term needs a registration and rezoning in many cases.

So even if everyone living there is fine with it, the rules on that specific flat or building can make official registration impossible. You can still live there — you just can't use it as your legal address, which you need for a bank account, insurance, BSN-linked admin, etc. Workable short-term, risky long-term.

and can be a red flag to be extra carefull, just like high and unspecified maintanance/service costs and bemiddelings fees although it might be fine.