i've read about this... this is the first time i have experienced it. what do i do?

kontact 5.7.3, kubuntu 18.04

i've read about this happening on this list many times, but as it never 
happened to me, i didn't pay attention to what to do to fix it.

i've been struck with the "Retrieving Folder Contents -- Please wait..." 
problem: i start contact and, instead of seeing individual messages, i 
see "Retrieving Folder Contents -- Please wait..." pretty much 
regardless of what i do.

it seems to go away, sometimes: i can start it, sometimes, without that 
message, but as soon as i move a message to a new location, or delete a 
message, i get "Retrieving Folder Contents -- Please wait...".

please help! 8/