This error happens when FIPS policy checking is enabled. You can tell whether it's enabled by checking the registry key here: HKLM\\System\\CurrentControlSet\\Control\\Lsa\\FIPSAlgorithmPolicy\\Enabled (1 is enabled and 0 is disabled).
If you're allowed to disable it, you can change the value of that key to 0. You can also try creating a config file in the same folder as the .exe with the same name as the .exe. For example, in C:\\Program Files (x86)\\Red Gate\\SQL Compare 10, create a file called RedGate.SQLCompare.UI.exe.config.
You'll need the config file to contain the following:
<?xml version="1.0" encoding="utf-8"?>
This might not work in certain locked down environments.
Hope that helps.