Hello Fabian,
what Kernel version are you using with the SAPKB70217? and what exact IE versions are you using?
Older IE browsers use Quirks mode v's newer IE Browsers which use standards mode(edge)
One can set the value of parameter sap-ie=edge or sap-ie=EmulateIE8
See the notes
1970427 - SAP GUI for HTML: Rendering problems in Enterprise Portal (EP)
1924378 - "SAP GUI for HTML: Info about standards and quirks rendering
modes".
Regards,
Oisin