Welcome to comminuties.
As per the error message that you are receiving "The endpoint certificate could not be validated due to exception Could not get certificate from host due to error", we understand the connection between Skyline and vCenter is broken because of the Thumbprint from the vCenter would have changed.
Request you to update the vCenter password on Skyline Collector. You could follow the steps mentioned below to update the steps:
Login to Skyline UI (https://SkylineIP/) using Admin
Click on Configuration
Click Edit on the vCenter which is having the errors.
Update the password
Click on update.
Thanks. Worked perfect