After upgrade, data loading activity is not able to open.
GDPR package is not been installed currently so is this is an impact?
Were there any errors in the postupgrade summary, and also if any, did you resolve the conflicts.
Can you clear your Client's cache and also install the same version as the new upgrade build, then create a new workflow and try the data loading activity once again.
Can you please specify the error(if possible a screenshot) you get while opening the data loading activity.
After upgrade there was a mismatch in workflow schema when compared to other instance. Adobe has changed schema it seems and now its working fine.