The login banner seems to fit into a uniformly sized box that scrolls if there is too much text for it. Windows disclaimer message size modification and the Ok button was mapped outside the view area.

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

A workaround is to choose a bigger window size(such as 800*600) for LogonUI(the default size is 600*520) by setting following registry values. 

To change the behavior from the default, choose the hooks you want to enable and add them together. Remember to use decimal mode when entering the value in the registry, or first convert to HEX.

For 64bit VDA: 
HKLM\Software\Wow6432node\Citrix\CtxHook\AppInit_DLLS\Multiple Monitor Hook\LogonUIWidth = DWORD:800 
HKLM\Software\Wow6432node\Citrix\CtxHook\AppInit_DLLS\Multiple Monitor Hook\LogonUIHeight = DWORD:600 

For 32bit VDA: 
HKLM\Software\Citrix\CtxHook\AppInit_DLLS\Multiple Monitor Hook\LogonUIWidth = DWORD:800 
HKLM\Software\Citrix\CtxHook\AppInit_DLLS\Multiple Monitor Hook\LogonUIHeight = DWORD:600 

Then reboot the VDA