I upgraded my firmware to the latest version 3.9.6.1_BT today and after this the mesh setup page shows the following error message.

Error : Invalid Basic Configurations. Please check Administrator Password and SSID/WPA Key.

Has anyone seen this error message before?

I've just seen this on the exact same setup, 3.9.6.1_BT on a 2862ac.

I've submitted a ticket to support, but not confident of a reliable response seeing as your query is from May (over 2 months ago).

I contacted support, from memory the admin password on the router was the issue, My old admin password for the router was 40 characters long and only had letter numbers and the hyphen character in it. Once I changed it then the Mesh error went away.

Yes it does seem to be admin password related. Mine included a "-" (hyphen/minus) character, as soon as I got rid of this everything worked. I found the same if the password contains the " (quotation) character or : (colon) character. Seems as though a lot of special characters are not allowed, but these are allowed and cause problems. Seems a little odd not to allow special characters in passwords.

I raised a ticket with support who are feeding this back to Taiwan. I think we should see it fixed in a firmware update soon.

I agree, such a simple thing to fix with character checking before accepting a password that can mess up other config.