Hello Paul,
Can you confirm your SAP BASIS and Kernel version (and patch level) that you are using.
Check if the following notes are relevant
1945330 - Controls on dynpro get lost after server communication
1958584 - WEBGUI: missing files in ITS.SAR archive of Kernel 721_EXT PL#201 on
Windows x64/ia64
Regards,
Oisin