remote: GitLab: You can only delete protected branches using the web interface

We are currently getting error messages from handling an MR in gitlab.

'git push --delete <gitlab server> pr/394' exited with status: 1
[stdout]
[stderr]
> Authorized uses only. All activity may be monitored and reported.
> remote: GitLab: You can only delete protected branches using the web interface.

Looking in the web interface, the branch is indeed still there and marked as PROTECTED.

Stacktrace:
at org.openjdk.skara.bots.notify/org.openjdk.skara.bots.notify.prbranch.PullRequestBranchNotifier.deleteBranch(PullRequestBranchNotifier.java:72)
at org.openjdk.skara.bots.notify/org.openjdk.skara.bots.notify.prbranch.PullRequestBranchNotifier.onStateChange(PullRequestBranchNotifier.java:87)
at org.openjdk.skara.bots.notify/org.openjdk.skara.bots.notify.PullRequestWorkItem.lambda$notifyStateChange$11(PullRequestWorkItem.java:193)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
at org.openjdk.skara.bots.notify/org.openjdk.skara.bots.notify.PullRequestWorkItem.notifyStateChange(PullRequestWorkItem.java:193)
at org.openjdk.skara.bots.notify/org.openjdk.skara.bots.notify.PullRequestWorkItem.run(PullRequestWorkItem.java:251)
at org.openjdk.skara.bot/org.openjdk.skara.bot.BotRunner$RunnableWorkItem.run(BotRunner.java:79)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:832)