401 or 404 when connecting to FME Server?

If I use a good host but with /fmeserver after the host name 

(ex. http://ap-fmeserv190/fmeserver)

Updating access token for ’test bad url for fmeserver
Request to update access token failed. HTTP Error: HTTP/1.1
404
-
http://ap-fmeserv190/fmeserver/fmetoken/generate.json

Updating access token for ’test bad url for fmeserver (ex. http://ap-iisarrfme/fmeserver) IIS & ARR - redirection

Updating access token for 'test bad url for fmeserver'
Request to update access token failed. HTTP Error: HTTP/1.1
404
-
http://ap-iisarrfme/fmeserver/fmetoken/generate.json

If I use good host name and really bad URL (ex. http://ap-fmeserv190/fmeserver/#/tokens/session)

Updating access token for ‘test bad url for fmeserver’
Request to update access token failed. HTTP Error: HTTP/1.1
401
-
http://ap-fmeserv190/fmeserver/#/tokens/session/fmetoken/generate.json

If I use a bad host name

Updating access token for ‘test bad url for fmeserver’
Request to update access token failed. HTTP Error:
Couldn’t resolve host name

Good Authentication 

If you do click the authenticate button and all is good with your hostname/url (ex. http://ap-fmeserv190) all you should see is:

Updating access token for ‘ap-fmeserv190’

Please share if you have error codes that you know about and see regularly.

Hello @abnanyazdani​ , would you be able to share the error dialog message? Here to help, Kailin

@fmeuser_gc​  We have your case! Thank you for filing it.

One thing that can happen is the use of https:// vs http:// but that is not your issue I don't believe.

1. If the FME Flow environment has been configured for https, then use https://<fully qualified domain name>.
2. You can test using the URL from the browser where you access the FME Flow Web UI and copy that into FME Form.
3. You can also try the hostname (test with both http & https) as this can sometimes help instead of using the FQDN. Occasionally users are not able to route directly to the hostname and must use FQDN. If this is the case, involve the Network Team and see if you can uncover any issues there.
4. Creating a new connection can resolve the issue... but if this issue persists, it is best to file a case with us (as you have done).  
5. There can be hiccups that are very intermittent. These are hard to diagnose or determine if it is Software or Environment at fault.