You could try below steps:
1. Check if this is due to any anti-virus software installed on the machine.
2. The minimum version required should match the version of the ESXi image/driver you are trying to import. For example: With Update Manager 6.5 you can upgrade hosts that are running ESXi 5.5 or ESXi 6.0 to ESXi 6.5. Host upgrades to ESXi 5.0, ESXi 5.1, ESXi 5.5, or ESXi 6.0 are not supported.
3. Ensure that the password of the username used for vCenter Update Manager installation has not been changed.
4. Ensure that the driver is compatible with the version.
If nothing works, try reinstalling the update manager.