Our system does not allow DOI numbers longer than 20 characters (including /). If we use longer ones, it just cuts them and places articles into wrong folders. Is it our mistake?
I’d recommend looking at the Crossref naming recommendations for DOIs. Constructing your identifiers – Support Center
DOIs don’t typically contain forward slashes as they are often construed with file paths. They are supported, technically, but they can lead to issues. However, I expect what you’re doing is putting a DOI suffix in the “Public ID” field. Public ID isn’t a DOI suffix. It’s like an extension on a file path you can write to link people to specific articles. These are distinct from DOIs, which are managed by the DOI and Crossref plugins.
I’d recommend also reading our Crossref guide to learn how to properly configure your DOI and Crossref plugin. Crossref OJS 3.2+ Manual