Backport command stopped working on Gitlab

It seems a new version of Gitlab is causing us to fail the /backport command. Here is the stack trace from the log:

org.openjdk.skara.network.UncheckedRestException: Request returned bad status: 415
at org.openjdk.skara.network/org.openjdk.skara.network.RestRequest.transformBadResponse(RestRequest.java:326)
at org.openjdk.skara.network/org.openjdk.skara.network.RestRequest.execute(RestRequest.java:403)
at org.openjdk.skara.network/org.openjdk.skara.network.RestRequest$QueryBuilder.execute(RestRequest.java:189)
at org.openjdk.skara.forge/org.openjdk.skara.forge.gitlab.GitLabRepository.addCollaborator(GitLabRepository.java:585)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.BackportCommand.handle(BackportCommand.java:168)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.CommitCommandWorkItem.processCommand(CommitCommandWorkItem.java:131)
at org.openjdk.skara.bots.pr/org.openjdk.skara.bots.pr.CommitCommandWorkItem.run(CommitCommandWorkItem.java:171)
at org.openjdk.skara.bot/org.openjdk.skara.bot.BotRunner$RunnableWorkItem.runMeasured(BotRunner.java:190)
at org.openjdk.skara.bot/org.openjdk.skara.bot.BotRunner$RunnableWorkItem.run(BotRunner.java:146)
at java.base/java.util.concurrent.Executors$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:831)

It looks like the rest request we use to add a collaborator to the fork repository is failing.