Hello,
Please in order to be able to progress on your issue can you provide these elements :
1. Relative file custom field configuration screenshot
2. A step by step to reproduce your current error to see if we are able to reproduce it on our localhost
3. On your side, can you reproduce your issue BUT with the Joomla error debug to level maximum, and provide the generated screenshot ?
Note : Global Configuration => System tab, then Debug System : "Yes"
AND Global Configuration => Server tab, then Error Reporting : "Development"
Awaiting your elements to progress on your subject.
Regards
Philippe
Team HikaShop