The ".LCK" file is a temporary file that is created while a team member is accessing the .PLP file (signing-in/out or sending/receiving changes). The file gets deleted after the access to the file has completed. If the person crashes during this access time the ".LCK" file will remain. You will need to manually delete the file (as long as you know that nobody else is accessing the file at that time).
Once the ".LCK" file is deleted, you should be able to get back into the file.
I haven't experience the latter file/message.
David Larrew, AIA, GDLA, GSRC
Architectural Technology Specialist
a r c h i S O L U T I O N S
WIN7-10/ OSX 10.15.7
AC 5.1-25 USA