The TIBCO Spotfire Server URL may fail to load in the browser, showing a "An internal error has occurred. Go to start page" error as below screenshot along with the following warning in the server.log even after successful server startup message in catalina log  ( from <TSS installation folder>\tomcat\logs).

This behavior is regardless of authentication type on Spotfire server.
server.log ( From <TSS installation folder>\tomcat\logs) captures warnings like “HTTP method GET is not allowed” or “HTTP method GET is not allowed”  : 

Example logs for HTTP method POST:

WARN 2020-02-28T16:49:05,493-0800 [unknown, #-, #261] server.security.HttpMethodsFilter: HTTP method POST is not allowed for URI /spotfire/nodemanager/trust/ocsp
WARN 2020-02-28T16:49:06,502-0800 [unknown, #1, #42] server.security.HttpMethodsFilter: HTTP method POST is not allowed for URI /spotfire/nodemanager/command
WARN 2020-02-28T16:49:43,540-0800 [unknown, #11, #336] server.security.HttpMethodsFilter: HTTP method POST is not allowed for URI /spotfire/nodemanager/messages
WARN 2020-02-28T16:49:06,502-0800 [] server.http.HttpCallable: Failed to send ServiceConfigCommand request to 'https://tss-kerberos1.boston.local:9403/spotfire/nodemanager/command' : 405 
org.springframework.web.client.HttpClientErrorException$MethodNotAllowed: 405 
    at org.springframework.web.client.HttpClientErrorException.create(HttpClientErrorException.java:87) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:122) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.DefaultResponseErrorHandler.handleError(DefaultResponseErrorHandler.java:102) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.ResponseErrorHandler.handleError(ResponseErrorHandler.java:63) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.RestTemplate.handleResponse(RestTemplate.java:778) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:736) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:670) ~[spring-web.jar:5.1.5.RELEASE]
    at org.springframework.web.client.RestTemplate.postForEntity(RestTemplate.java:445) ~[spring-web.jar:5.1.5.RELEASE]
    at com.spotfire.server.http.HttpCallable.call(HttpCallable.java:108) ~[spotfire-node-manager.jar:?]
    at com.spotfire.server.http.HttpCallable.call(HttpCallable.java:36) ~[spotfire-node-manager.jar:?]
    at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_201]
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) ~[?:1.8.0_201]
    at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) ~[?:1.8.0_201]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:1.8.0_201]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:1.8.0_201]
    at java.lang.Thread.run(Thread.java:748) [?:1.8.0_201]

Example logs for HTTP method GET:

WARN 2020-02-28T16:52:10,774-0800 [unknown, #9, #256] server.security.HttpMethodsFilter: HTTP method GET is not allowed for URI /spotfire/
WARN 2020-02-28T16:52:10,821-0800 [unknown, #9, #257] server.security.HttpMethodsFilter: HTTP method GET is not allowed for URI /spotfire/resources/min/app-lib.css
WARN 2020-02-28T16:52:10,821-0800 [unknown, #9, #258] server.security.HttpMethodsFilter: HTTP method GET is not allowed for URI /spotfire/resources/min/third-party.js
WARN 2020-02-28T16:52:10,821-0800 [unknown, #9, #259] server.security.HttpMethodsFilter: HTTP method GET is not allowed for URI /spotfire/resources/min/app.css
WARN 2020-02-28T16:52:10,821-0800 [unknown, #9, #260] server.security.HttpMethodsFilter: HTTP method GET is not allowed for URI /spotfire/resources/min/app.js

  <allowed-http-methods>       <method-name>DELETE</method-name>       <method-name>GET</method-name>       <method-name>HEAD</method-name>       <method-name>PUT</method-name> </allowed-http-methods> 

• Export currently used configuration from the Spotfire database. To export, either login to uiconfig.bat file (uiconfig.sh on Linux) which is located in the <Installation dir>\tomcat\spotfire-bin directory  OR  execute CLI command export-config  
• Open exported server configuration from given location in text or xml editor and search for <allowed-http-methods> tag. Ideally it should have below mentioned five http methods :     

   <allowed-http-methods>                    <method-name>DELETE</method-name>                    <method-name>GET</method-name>                    <method-name>HEAD</method-name>                    <method-name>POST</method-name>                    <method-name>PUT</method-name>             </allowed-http-methods>

• Add any missing http methods in your configuration manually
• Save the changes and import the configuration back to the database, either using uiconfig tool or use import-config command
• Restart the Spotfire server
• After successful server startup message in catalina logs, try opening the Spotfire server url in browser                                           

• https://docs.tibco.com/pub/spotfire_server/10.3.6/doc/html/TIB_sfire_server_tsas_admin_help/GUID-3D77DED9-B16F-41F3-8D82-DE3FBF5DD09F.html

• https://docs.tibco.com/pub/spotfire_server/10.3.6/doc/html/TIB_sfire_server_tsas_admin_help/GUID-3188EF2A-E7BD-481C-822B-7579B69B0DB9.html

• https://docs.tibco.com/pub/spotfire_server/10.3.6/doc/html/TIB_sfire_server_tsas_admin_help/GUID-AD39E75E-9C03-41A1-A2F1-01363389A398.html