This could be totally unrelated, but I had the same problem after upgrading a pair of 15400's to R80.20 last week. We worked extensively with TAC and identified a hotfix to resolve the issue. I was advised by TAC yesterday that this hotfix is included in the latest Ongoing Take (Take 73) of R80.20.
Obviously there could be huge differences since your issue is on Open Server and mine was on an Appliance, but it couldn't hurt to consider the Ongoing Take before reverting back?
https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solut...
If you choose to open a TAC case (not the worst idea), you may want to familiarize yourself with this SK, as you will likely need to invoke kdb mode on the Gateway to capture the crash information. We were finding that in the event of a total system freeze, no relevant logs were being generated.
https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solut...
R80 CCSA / CCSE