Unable to Store Credentials with secured feature service

[AllenScully2]

Attempting to add  ArcGIS Server 10.9.1 secured feature service to AGOL as an item, using URL.  

I can add the service to a map, enter credentials, and they are accepted, save the map, no issues.  But when I add the item and say 'save credentials' it fails, says invalid user name or password.  The error details are attached.

We have investigated ArcServer config, webadaptor and IIS settings, reverse proxy settings - to the best of our knowledge this should work - and we use this fairly simple workflow on our current production environment (which is 10.7.1) - So I am left to wonder if this is a bug for 10.9.1, or something with our reverse proxy passing credentials?  But you CAN pass credentials, AGOL just can't save them.  

Thanks -

Allen

[PierreloupDucroix]

I had the same behavior.

The password used by my user did not allow storing credentials, with the message 'An error has occurred'.

Creating a new AGOL user with strong password resolved the issue.