Hi Ewan and welcome to Nimble.
This can be caused by a number of possibilities, I would suggest creating a support ticket with Nimble Support so they can investigate the error. Depending on your location you can find the correct toll free number here Customer Support | Nimble Storage.
One possibility for this error is the use of a SSL Certificate on the vCenter server having an RSA key size <1024 bits. You can check this on the vCenter Server by using a browsing to the vCenter Web Client and viewing the certificate. You want to ensure the key size is larger than 1024 bits, if not it will need to be regenerated.
I hope this helps.