Had this issue today. I had built the IGEL UMS and imported my IGEL thin clients, however the UMS became unstable so I had to rebuild. Once I had rebuilt the UMS I went to scan and import the IGEL thin clients only to find that they could not be imported due to Error: Invalid Certificate.
The quickest way to resolve my issue was a factory reset of the thin clients as I had not rolled out any profiles to them. To hard reset the IGEL client you must be at the machine. Turn it of then turn it back on. Keep hitting ESC until you see the screen below.
Then select Reset to factory defaults.
The thin client will continue to boot until you see the screen below. Select yes to confirm the factory reset.
Warning, this removes all config locally so if you have profiles setup IT WILL REMOVE THEM. Once done the client will reboot and you import the client into the UMS and deploy profiles