Over 30 years renting apartments, suddenly no building permit

Since the 1980s, a building with rental apartments has been rented out in Austria/Styria; until the early 1990s, the property was renovated and prepared. Tenants have repeatedly registered/deregistered with the municipality. The assignment of property numbers was never disputed, nor was the registration/deregistration ever not carried out. Now, suddenly, building permits are said to be missing, and it is no longer possible to register tenants for vacant apartments? Unfortunately, I do not have all the old plans for the building myself. There was no notification from the municipality; tenants were simply told: "You cannot register for this unit!" The tenant then called me. I was only told that only about half of the apartments were listed in the submission plans?!? Does it not matter that different tenants have been legally registered in these apartments with a registration certificate at the municipality for more than 30 years? What steps do I need to take now? What sanctions should I expect? How am I supposed to comply with the registration law if I cannot register my tenants? I thank you in advance for helpful answers.

You need a lawyer. Or you have to settle it yourself at the [Behörde].

You need a lawyer. Or you have to resolve it yourself at the authority.

Thanks, but can you tell me what to expect. After all, this is already strange and shouldn't the municipality at least give a hint rather than waiting until a tenant tries to register. I find this approach highly damaging to business. After all, the tenant contacted me and not the official.