Sys is undefined, Type is undefined

Problem
So today I came across a strange error in SharePoint/Project Online.
3 users were not able to see the Project Center, Resource Center or add new items to a document library.
The issue was clearly related to their PC's as other users had the same problem on their PC's and they didn't have the problem on other PC's.
Everybody had the same version of Internet Explorer, Same regional settings and same language installed.

In their IE log they were all receiving the following error.

 SCRIPT5009: 'Sys' is undefined   
 projects.aspx, line 124 character 219  
 SCRIPT5009: 'Sys' is undefined   
 projects.aspx, line 150 character 1  
 SCRIPT5007: The value of the property 'WebForm_InitCallback' is null or undefined, not a Function object   
 projects.aspx, line 656 character 1  
 SCRIPT5007: The value of the property '$get' is null or undefined, not a Function object   
 shell.js?rev=YwnNMDxuVwsNnI9A1oL38w%3D%3D, line 2 character 50132  
 SCRIPT5009: 'Type' is undefined   
 sp.core.js?rev=7ByNlH%2BvcgRJg%2BRCctdC0w%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 sp.ui.dialog.js?rev=IuXtJ2CrScK6oX4zOTTy%2BA%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 sp.runtime.js?rev=9sKdsC9N6p2BiRk3313M7Q%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 sp.js?rev=SpGB4%2FzYmCWpwoPWNG2dsg%3D%3D, line 2 character 1  
 SCRIPT438: Object doesn't support property or method 'registerInterface'   
 cui.js?rev=k%2B4HtUzT9%2B3mSycgD7gPaQ%3D%3D, line 2 character 132  
 SCRIPT5009: 'Type' is undefined   
 sp.ribbon.js?rev=F%2BUEJ66rbXzSvpf7nN69wQ%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 projectserverscripts.js?rev=wLWEB16RvL7LvVwBTEDczw%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 pagepropertymgr.js?rev=MSfviAZ5MiJZfNsfZFcIEw%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 remotetextconv.js?rev=SvbL5%2Fn%2FylnTsF%2FASnvAng%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 webmethodmanager.js?rev=QZWPalD63GAq0Ux6QiTvww%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 projectframework.js?rev=5poW1un5idh68Af5W7%2By%2FA%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 gridsatellite.js?rev=OD4g9TDABejWnsQti9BHCg%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 utility.js?rev=6L1OxKcWB8r9YEeRFq7iYQ%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 dom.js?rev=f7dn0H6JVBq%2BJjo%2Fz2ZJRA%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 projectcentersatellite.js?rev=hlKj0a968OT2D7gm%2FuKlMQ%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 sp.ui.combobox.js?rev=166SKYsDoffDAabXKBONxw%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 jsgrid.js?rev=mx0FnsqZMQTgnLRUymCf8w%3D%3D, line 2 character 1  
 SCRIPT5009: 'Type' is undefined   
 sp.datetimeutil.js?rev=c1q2E%2FwVfTjFPk%2BMTmPvDg%3D%3D, line 2 character 1  
 SCRIPT5007: Unable to set value of the property 'GanttPane': object is null or undefined   
 jsgrid.gantt.js?rev=QeMlynX3yiGsrTWH8GAGQg%3D%3D, line 2 character 1  
 SCRIPT5009: 'WebForm_SaveScrollPositionSubmit' is undefined   
 projects.aspx, line 832 character 1  
 SCRIPT5007: The value of the property '$get' is null or undefined, not a Function object   
 shell.js?rev=YwnNMDxuVwsNnI9A1oL38w%3D%3D, line 2 character 5898  
 SCRIPT5007: The value of the property '$addHandler' is null or undefined, not a Function object   
 shell.js?rev=YwnNMDxuVwsNnI9A1oL38w%3D%3D, line 2 character 1727  

This didn't tell me much other than it looks like something is broken in IE. Google and Bing gave a lot of results, but all of them were  related to programming and ASP.Net. I could not find much related to SharePoint.

Solution
After a lot of digging and trying tried something that wasn't related to any of the errors but was a pure guess.I tried to disable the ASP.Net Compression.

This immediately solved the problem.

I still don't know what is the root cause, but I guess it is an error in one of the files being updated on the client PC's.