Scopus/Crossref Plugin - does not show results - OJS 3.1.2.1

I have installed this plugin from plugin library. This is a multi-journal system.
When there is not any citation it works:
https://doi.org/10.14232/actahisp.2021.0.49-73

But when there are citations (checked in Dimensions) it shows nothing. A short flash then nothing.
https://doi.org/10.14232/actahisp.2000.5.83-101
We are taking part in the cited-by at Crossref, and we have another standalone but upgraded OJS where this plugin works like a charm. (After a short discussion with Crossref)

Hi @molnart,

Is there a reason that you’re staying at 3.1 and not upgrading?
3.1 no longer supported by PKP. I would recommend that you upgrade to the newest version of OJS, as it is possible that your issue will be resolved by upgrading. However, other community members may wish to offer assistance and advice.

-Roger
PKP Team

Hi, @rcgilis,
There are several reasons, we have a multijournal system, with many different editorships, and it very hard to organize to not disturb their processes. The IT department is had been outsourced from our library, so we have a less impact on the upgrade. I hope that @RBoelter will see this post.

Hello @molnart,
sorry for the late reply, but I was on vacation.
Seems like there is a problem with the article list. If you disable the "cited by" article listing option in the plugin settings, there should be a result (without the article list).
I can’t reproduce this problem because the plugin still works fine with all our 3.1.x journals. It would be helpful if you could send me the error log, because I am not able to check your crossref records via the API, as I get the error “not authorized to view references for DOI 10.14232/actahisp.2000.5.83-101 owned by 10.14232”.