Another workaround that *should* work is to remove the following line from weewx.conf:
lang = en
A workaround that only works if your language is ‘en’.
The code in reportengine only looks for the file if lang is specified.
I don’t have a lang = en line in my weewx.conf and have no such messages in the log for my enabled Rsync skin.
I guess it is not necessary to specify a language for rsync. May be there a "lang = en" line direct under [StdReport]. A work around could be to create an empty file with the name that is missing.