3433352 - Invalid proxy response when trying to connect BAS to on-premise system

When trying to connect SAP Business Application Studio to an on-premise system (such as in an Adaptation Project), the following symptoms are experienced:

• No data available when trying to select your application.
• Request failed with status code 503 when selecting your on-premise system.
• The SAP Cloud Connector traces show:
  #DEBUG#com.sap.core.connectivity.tunnel.client.AbstractClient#Thread-663#          #Connecting to address connectivity.<region>.applicationstudio.cloud.sap:443 using proxy proxy/xxx.xxx.xxx.xxx:3128. Proxy user is
  #ERROR#com.sap.core.connectivity.tunnel.client.handshake.AbstractClientHandshaker#tunnel-client-14-2#          #Invalid proxy response: DefaultHttpResponse(decodeResult: success, version: HTTP/1.1)
  HTTP/1.1 403 Forbidden
  Server: squid/4.17
  Mime-Version: 1.0
  Date: <date>
  Content-Type: text/html;charset=utf-8
  Content-Length: 3651
  X-Squid-Error: ERR_ACCESS_DENIED 0
  Vary: Accept-Language
  Content-Language: en
  X-Cache: MISS from xxxxxxxxxxxxxxx
  X-Cache-Lookup: NONE from xxxxxxxxxxxxxxx:3128
  Via: 1.1 xxxxxxxxxxxxxxx (squid/4.17)
  Connection: keep-alive
  

• SAP Business Application Studio.
• SAP Cloud Connector.

squid, proxy, 403, forbidden, invalid proxy response, BAS, cloud connector, connect, adaptation project, 503, no data available, request failed. , KBA , CA-BAS-SRVC , Consume SAP Services - SAP Business Application Studio , Problem