Initial (working) configuration: The job is configured to be triggered by SCM changes using the Git plugin. Branch specifier is "**" (= any branch). So far, this works as expected. SCM is queried regularly and the build is triggered if new commits are found.

Then I checked the "build is parameterized" option and added a parameter "BRANCH" with default value "**". This doesn't break things yet. BUT when I now configure this parameter "${BRANCH}" as branch specifier in the check SCM build step, the build is triggered EVERY time, even if there are no new changes.

The log says the Build was triggered by SCM changes. But it also says that there were no changes.