An issue has been reported in Reckon APS Tax 2019.1.0 where the following error triggered in some amended returns:

Amended document cannot be opened following creation or it is in read-only mode

This issue is caused by the corrupted data in the shared rental or assets. The workaround is to set the original return back to Started. Please make sure you save a copy of return before turning the status to Started in case any data becomes missing.