Hi Tim,
Then I would propose to check if the request reaches the server node (Java process). This could be done by collecting traces with the security troubleshooting wizard - see Collecting Traces for Troubleshooting Security Problems - System Security - SAP Library. You may select incident type "Authentication", start the tool, access /otpadmin alias, stop the tool. In the generated traces you should see records like this if the request has reached the server:
CLIENT: 9842 [a.b.c.d : 55187], REQUEST:
GET /otpadmin HTTP/1.1
accept: text/html, application/xhtml+xml, */*
accept-language: en-US,en;q=0.7,bg;q=0.3
user-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
accept-encoding: gzip, deflate
host: otp.acme.com
dnt: 1
connection: Keep-Alive
CLIENT: 9842 [a.b.c.d : 55187], REPLY:
HTTP/1.1 302 Found
Location: https://otp.acme.com/otpadmin/
Server: SAP NetWeaver Application Server 7.20 / AS Java 7.31
Content-Length: 0
Date: Wed, 16 Mar 2016 12:57:13 GMT
CLIENT: 9843 [a.b.c.d : 55187], REQUEST:
GET /otpadmin/ HTTP/1.1
accept: text/html, application/xhtml+xml, */*
accept-language: en-US,en;q=0.7,bg;q=0.3
user-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
accept-encoding: gzip, deflate
host: otp.acme.com
connection: Keep-Alive
dnt: 1
CLIENT: 9843 [a.b.c.d : 55187], REPLY:
HTTP/1.1 302 Found
Location: https://otp.acme.com/otpadmin/index.jsp
Server: SAP NetWeaver Application Server 7.20 / AS Java 7.31
Content-Length: 0
Date: Wed, 16 Mar 2016 12:57:13 GMT
CLIENT: 9844 [a.b.c.d : 55187], REQUEST:
GET /otpadmin/index.jsp HTTP/1.1
accept: text/html, application/xhtml+xml, */*
accept-language: en-US,en;q=0.7,bg;q=0.3
user-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
accept-encoding: gzip, deflate
host: otp.acme.com
connection: Keep-Alive
dnt: 1
CLIENT: 9844 [a.b.c.d : 55187], REPLY:
HTTP/1.1 302 Found
Server: SAP NetWeaver Application Server 7.20 / AS Java 7.31
Content-Type: text/html;charset=UTF-8
Location: https://otp.acme.com:443/webdynpro/resources/sap.com/sso~otp~adm~wd/AdminApplication
Content-Encoding: gzip
Content-Length: 727
Date: Wed, 16 Mar 2016 12:57:13 GMT
CLIENT: 9845 [a.b.c.d : 55187], REQUEST:
GET /webdynpro/resources/sap.com/sso~otp~adm~wd/AdminApplication HTTP/1.1
accept: text/html, application/xhtml+xml, */*
accept-language: en-US,en;q=0.7,bg;q=0.3
user-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
accept-encoding: gzip, deflate
host: otp.acme.com
connection: Keep-Alive
dnt: 1