With Internet Explorer 11 Microsoft extended their Protected Mode feature with an Enhanced Protected Mode option that can be enabled in the enhanced Internet Explorer settings. By default, the Enhanced Protected Mode option is NOT active. If the Enhanced Protected Mode is active, then Internet Explorer will crash when Adobe Flash is loaded and plays a video. This issue is widely reported from people surfing YouTube.
In a Citrix environments, you should have restricted and configure Microsoft Internet Explorer through group policies and with IE 11 done in Preferences | Control Panel Settings | Internet Settings (configured from at least Server 2012, need IE 10 settings). For some unknown reason, Microsoft enables the Enhanced Protected Mode when creating a new configuration for Internet Explorer and will then cause the crash when using Adobe Flash.
This issue might be fixed in the future either by Microsoft or Adobe but for now, you should disable the Enhanced Protected Mode as it is without policies.