@Czega 

What switch do you have?

  @Czega 

It is very poorly documented how many VLAN switches support, but I believe it is around 256 on the SG2xxxx switches. So you have probably hit a limit. There was something about this on the forum a while ago but I can't find this post again.

  @MR.S 

Thanks for your quick response.

Switches:

SG2218P
SG2428P
SG3428
SG3452

I don't have acces to the controller right now but it seemed that the error was not related to a specific switch but the controller itself. Of course, I understand that the knows what switches it has.

There are quite a few SG2XXX switches in the network so we won't be able to replace them with more expensive SG3XXX ones.

Thank you,

Peter

  @Czega 

I tried this, I created an empty site without any switches and I was only allowed to create 256 vlans, I have a windows software controller.
I couldn't create vlan 257

so I don't know where the limitation is, here the message says that it is the gateway that is the problem but I have no gateway in this empty site.

  @MR.S 

I can't acces the actual OC but here's a snip from another one:

We created the VLANs like above, so "Gateways and switches". Would make any difference to choose "Switches only"?

To tell you the truth I don't know what this really is but routing is done by a Mikrotik CCR, we use only switches from TP-Link so I think we could try Switches only.
I will be able to test on on Monday. Maybe could you try this in the meanwhile?

Thank you so much,

Peter

  @Czega 

If you use switch only, you can create multiple VLANs. I created 512 VLAN

  @MR.S 
Thank you very much for valuable help, I will test it out on Monday.

BR,

Peter

I tested it, works just fine!

Thank you once again!

BR,

Peter