Scan for Bitbucket yaml repository fails with unknown error

[Andreas]

I am running bitbucket server, and bamboo server.

We have deposited a yaml build spec on the bitbucket server, and linked bitbucket with bamboo.

We had minor issues with this link, because bamboo is behind an IIS as a reverse proxy (this was necessary to enable https).

In any case: the link is working. I can generate a linked repository, and I can select a branch within any repository.

However, when I scan for build specs, I get the following:

Couldn't start Bamboo Specs scanning

An unknown error has occurred. See the logs for more details.

However, I do not see any log messages that seem directly related. Only one log-message is created, and it supposedly a warning:

2020-07-10 21:56:34,331 WARN [http-nio-8085-exec-15] [XsrfResourceFilter] Additional XSRF checks failed for request: http://localhost:8085/rest/api/latest/repository/1638401/scanNow , origin: https://bamboo.XXXX.com , referrer: https://bamboo.XXXX.com/admin/configureLinkedRepositories!doDefault.action , credentials in request: true , allowed via CORS: false

Any hints or tips? Thanks!