Hi julesmalcomson
First get the newest build from https://www.capturingreality.com/My
Second set CACHE folder location to say D:/RC_temp folder ( WORKFLOW->settings-> CACHE LOCATION ( custom location ) )
Look if have not to deep path and no special characters in path when saving ( Windows problem with 255+ characters in path )
the missing files can be related to this eventually if have some disk space cleaning app that could delete RC CACHE files.
Report back if stil cannot get it working.