Hi @Feline Lemant ,
Here are some steps you can take to resolve the "failed on start" error with your SharePoint 2019 workflows:
1.Deactivate and re-activate the Workflows feature in Site collection features and check again.
2.Strat workflow with other account instead of system account and compare the results.
3.Check ULS log to find the detailed information.
4.If it's convenient, you can provide the detailed steps of your workflow, which can further help you test and troubleshoot the issue.
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