As I'm not able to give you the why and how, the Docman team kindly accept to explain where is the issue in Flexicontent, I copy paste their answer :
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Hello Xavier,
Error is =>
drops.arunasmazeika.com/Screen-Shot-2023-11-27-12-16-48.42.png
,
This is not something you should ever do ... I bet that if you contact the Joomla Extension Directory they might even consider taking this extension down, this could be considered a violation of their terms for remaining listed. This could easily break extensions that rely on a given API based on that class existence, like we do ... DOCman supports Joomla 3, 4 and 5, not an easy feat, so we make some checks against old classes like this one.
Again this is a re-definition of a legacy Joomla core class which results on an API change ... we are using that class if we find it and then assume that the object actually implements a given interface ... this is disguising another different object using that class name! this is really bad bad bad!
Please show them the screenshot posted above and see if they are willing to do something.
This is also to read to complete this :
github.com/FLEXIcontent/flexicontent-cck...stants.php#L106-L116
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For developers, all is explain on what to fix in Flexicontent.
Many thanks to Docman team to identify this and why this is an issue in Flexicontent code for J4 and further.
Xavier
