This is how we fixed this issue in our side:
1. Open the \Engine\Config\BaseEngine.ini file
2. Search for the [Zen.AutoLaunch] category and after the “DataPath=” folder path use the old "%APPSETTINGSDIR%Zen/Data" format or any custom path access like “DataPath=%ENGINEDIR%Zen/Data” for example
3. Done, try to open a 5.4 project
Epic changed the Zen Store path access to a new one in 5.4 for some reasons and this causes problems for some people like us, so this workaround could work.