Studio cannot establish server connection (404)

[realbeat]

Hello

JasperReports Server 7.2.0 Professional.
JasperReports Studio 7.3.1

After adding an custom external authentication (SAMLv2 based) on JasperReports Server we cannot connect anymore with Studio. Reason: 404
I see the 2 trial requests submitted by Studio in the access log:
    "GET /jasperserver-pro/j_spring_security_check?forceDefaultRedirect=false&j_password=[sNIP]&j_username=superuser HTTP/1.1" 404 58274
    "POST /jasperserver-pro/services/repository HTTP/1.1" 404 58268

• j_spring_security_check is deprecated with Spring-security 4. Why is studio still using this endpoint?
• the second is using SOAP endpoint, which has been removed with some older version

Turned out, that the authentication is successful, but since the dispatcher does not find any page for j_spring_security_check, a 404 "not found" is returned and Studio cannot  handle it.

But why does it work without the custom external authentication??
Because deprecated MetadataAuthenticationProcessingFilter is executed conditionally execution conditionally
"on the absence of proxyAuthenticationProcessingFilter bean.  The latter would mean that SSO is configured the old way, and the filter will be executed as pre-5.1."

So, by introducing the external authentication, as a side effect you break the internal authentication :-(

I googled and tried a lot of things without success. E.g. configuring static successHandler redirecting to flow.html did not work.
How can I make the internal authentication work again?
Any hints would be greatly appreciated.

regards
Beat

PS: I did not go into details with configs etc, since it's a very conceptional problem. Some people must have encountered this before.