The following errors/Event IDs occur because my XenApp 6.5 Farm/Setup lost the Citrix Start-up License|Server License. After re-adding, it works again.
CITRIX WEB INTERFACE
EVENT ID 30001
Site path: C:\inetpub\wwwroot\Citrix\XenApp.
An error occurred while attempting to read information from the Citrix servers: An existing connection was forcibly closed by the remote host. This message was reported from the XML Service at address http://XXXXXXXXXX:8080/scripts/wpnbr.dll [com.citrix.xml.NFuseProtocol.RequestAddress]. The specified Citrix XML Service could not be contacted and has been temporarily removed from the list of active services. [Unique Log ID: XXXXXXXX]
For specific information about this message, see the Web Interface documentation at http://support.citrix.com/proddocs/topic/web-interface-impington/wi-log-messages-event-ids-hardwick.html.
EVENT ID 31003
Site path: C:\inetpub\wwwroot\Citrix\XenApp.
All the Citrix XML Services configured for farm xenapp failed to respond to this XML Service transaction. [Unique Log ID: XXXXXXXX]
For specific information about this message, see the Web Interface documentation at http://support.citrix.com/proddocs/topic/web-interface-impington/wi-log-messages-event-ids-hardwick.html.
ZDC
EVENT ID 10001
A usable server cannot be found on which to launch the application. Application: WordPad, Client: XXXX.-XXXX-XXXX–nshic (address: X.X.X.X;;;), User XXXX.XXXX. Check your worker group definitions and load balancing policies to verify appropriate servers are assigned for WordPad.
QFARM /LOAD = Server Load: 20000
Links that helped me on the way to the solution resp. some other solutions based on the Errors/Event ID:
Application Launch Requests Might Fail on a Provisioned XenApp Version 6.0 or 6.5 Multihomed Server
A usable server cannot be found.. Citrix XenApp 6.5
Unable to Launch Published Applications from XenApp 6.x
ComputerComputerComputer 