issues on premise moment

Hi all,

I have the following problem. Can anyone think along with me, and push me in the right direction?

We have an app deployed that is on premise at the customer. Latest version of both gui and IAM. Everything works fine on my pc, and uat environment, but the customer environment throws this 405 error. 

See here: 

Any advice on this? I would like to advice the customer on what to change here;

Thanks! 

Blommetje

(title is weird - had issues with the forbidden keywords. Found out they we in the screenshot through Teams - but can’t change it! Should be of course ‘on-premise’)