I d'like to share my finding and hope that someone could figure out why it happens as it does.
I also had troubles enabling TKIP on an SSID.
1> When creating an SSID with the wizard with WPA/TKIP auth/encryption: An error message is displayed on the GUI when quitting the SSID config page:
Also note the absence of a encryption selection:
This is due to the fact that the wizard selects TKIP only, and that is not a valid encryption combination.
To find out what are valid combinations, see this post:
What-are-the-valid-opmode-combinations
To solve this error message, simply select an extra encryption on the 'Advanced' tab to obtain a valid pair (i.e. wpa2-psk-aes)
Now there will be a valid 'mixed encryption mode' and no more error messages:
2> If you want to add TKIP to an existing SSID with i.e. WPA2-PSK It matters how you do it.
If you try to select mixed mode and a valid combination, there will be an error message:
To prevent this I found that the correct extra encryption type had to be selected from the 'Advanced' tab:
After applying this, there is no error and a valid mixed mode is selected on the 'Basic' tab:
Rgds
Peter