2224052 - 503 Service not available - IcrFindTargetSystem: No system found for

The service is available and accessible via direct access to the back-end system, however the HTTP error 503 Service not available appears when accessing via SAP Web Dispatcher.

When reproducing the issue via the SAP Web Dispatcher (dev_webdisp) trace level set to 2 or 3, following entries are found:

[Thr ...] IcrFindTargetSystem: No system found for  <url / vhost / path>

[Thr ...] HttpGetRouteTargetSystem: no system available, rc=-2

[Thr ...] No target system found

or

[Thr ...] Request could not be dispatched to system [SID]: no valid server found. Check connectivity to backend servers.

or

[Thr ...] PlugInStopConn: role: Server(1), error: No system found(-51), write_err: 1
[Thr ...] PlugInStopConn: write error response
[Thr ...] HTTP [<connection id>] error:  ICMENOSYSTEMFOUND (No system found); No target system found

or

[Thr ...] *** ERROR => HttpGetCgpVirtHost: error No data available(-15) during virtual host lookup [icxxvhostuti 257]

Together with above trace entries, instead of "503 Service not available" message in the browser, following message can apear instead:

Unser Service ist zurzeit leider nicht verfuegbar. Bitte versuchen Sie es spaeter noch einmal.
Wir entschuldigen uns für Unannehmlichkeiten

Our service is not available at the moment. Please try again later. 
We apologize for the trouble caused.

A Web Dispatcher trace at level 2 shows the following for the SAML SLO callback URL:

GET /sap/saml2/sp/slo/100?SAMLRequest=...
IcmLookupVhostTable: urlprefix=/sap/saml2/sp/slo/100 ... rc=-15
IcrFindTargetSystem: No system found for vhost: <hostname> url: /sap/saml2/sp/slo/100

• SAP Web Dispatcher
• Operating System independent
• Database independent
• SAP NetWeaver based system
• ABAP Platform based system

wdisp/system, icm/HTTP/mod, SRCSRV, SRCVHOST, SRCURL, 503 Service not available, Not Reachable,sap/bc/,/ui2/nwbc/ , KBA , BC-CST-WDP , Web Dispatcher , BC-JAS-COR , Enterprise Runtime, Core J2EE Framework , BC-CST , Client/Server Technology , BC-MID-SCC , SAP Cloud Connector On-Demand/On-Premise Connectivity , Problem