I and a fellow Yunohoster installed Calckey instances and face same issue of unable to upload lists downloaded from our previous instances. The bug is logged here: #9999 - [Bug]: Unable to import following.csv - calckey - Codeberg.org
So if you are planning to migrate people from another instance and you have large lists to import, you might want to wait until this is fixed and packaged for upgrade.
My YunoHost server
ARM board and X86 laptop.