One thing that can happen is that within the first 24 hours of enabling SSL Inspection, some of our processes for refreshing trusted CAs might take some time to complete, as a result some websites might respond with a browser error. So I recommend to you to wait 24 hours after enabling Full SSL Inspection in order to have that thing completed.
But if the case does not relate to SSL Inspection at all and you're using Basic SSL Inspection, then this is another story.. and for that, please open a support ticket so that Check Point Support can look at this more closely. See https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solut...
Update: the issue with SSL Inspection and untrusted websites has been fixed as of May 6, 2021. For more, see Harmony Connect - What's New https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solut...
