OAuth login does not work when trying to upload changeset

[floscher]

I tried to upload a changeset with a JOSM instance, in which I wasn't already logged in.

When JOSM tried to upload I was prompted to authenticate. I chose OAuth authentication in semi-automatic mode.

I then clicked "Retrieve Request Token". A dialog with a indeterminate progress bar appears and disappears, but no browser window opens.

Expected behaviour:
A browser window opens with the URL ​https://www.openstreetmap.org/oauth/authorize?oauth_token=some_oauth_token

When I go into Preferences › Connection settings › Authentication › Authorize now everything works as expected. Only when triggered by an upload I can reproduce the faulty behaviour. Also the fully automatic authentication works in both cases.

[floscher]

The request token is retrieved successfully:
2018-04-17 12:34:56.789 INFORMATION: GET https://www.openstreetmap.org/oauth/request_token -> 200 (124 B)
But neither the UI is updated, nor is an exception thrown.

[floscher]

Closed as duplicate of #14091.