Hi @TPComputer 

To better understand the issue, please share with us the following info:

1. a screenshot of the Device page so we could know what kind of Omada SDN EAP devices you are using;

2. A screenshot of the error message page;

3. A screenshot of the SSID config page.

4. What kind of controller you are using? (you choose both cloud-based controller and software controller for this thread).

  @Vincent-TP 

  @TPComputer 

Hi Vincent,

here is my Devices Page

And here is the SSID Page. 
I create a new WLAN group and copy all SSIDs from the default group. When I do this, the message "The SSID "TPC 5G, TPC 5G Terasse" is illegal and will be deleted."  appears. 

I'm using the cloud-based and the software-based controller for testing. The error occurs with both.
Thank you for your support

Thomas

 Hi @TPComputer 

We can create 8 SSIDs for each band.

How many SSIDs have you created already?

Why do you need so many SSIDs? If not necessary, it's not recommended.
 

  @Vincent-TP 

I have 4 SSIDs at 2.4 Ghz and 4 SSids on 5 Ghz.

Hi  @TPComputer 

Thanks for your reply.

If you try to create a new SSID named  "TEST", will it succeed?

  @Vincent-TP 

Yes, it Works

  @TPComputer 

Thank you so much for taking the time to post the issue on TP-Link community!
To better assist you, I've created a support ticket via your registered email address, and escalated it to our support engineer to look into the issue. The ticket ID is TKID241246500, please check your email box and ensure the support email is well received. Thanks!
Once the issue is addressed or resolved, welcome to update this topic thread with your solution to help others who may encounter the same issue as you did.
Many thanks for your great cooperation and patience!