Related Lists never load fully.

1. Related lists keep loading and never loads fully.
2. Even when all the related lists lists are loaded, you will see message at the bottom of the form "Related lists are loading".

All versions.

1. Customized UI Action with 'current.update' in the script for one of the related lists with empty records.
   Since there are no records in the related lists section , current.update in the script, looks for current record on the empty table, causes error and that causes related lists to never fully load.
2. List action is true for that UI Action.

Setting the list action to false, for the custom UI Actions with list action true, will resolve the issue. To set it to false, follow below steps:

1. Navigate to UI Action, open the required record.
2. Uncheck the box for 'list choice' and it will make list action false.

There can be different reasons, related to customized UI Actions, other than the mentioned one causing the issue.