Opening and saving files on a NAS - Error with opening file, No such file of directory

I have a problem with opening and saving files on a NAS.

When using the RStudio 'open an existing file' icon for a project on my NAS I got correct lists of NAS files. However an attempt to open one of these files fails sometimes and results in the message "Error with opening file, No such file of directory".
And when RStudio did succeed in opening a file, an attempt to save it may result in the message "Error saving file, No such file of directory"

I am using the latest versions: MaxOS High Sierra 10.13.6, a Synology NAS with DSM 6.2-23739 Update 2 and RStudio Version 1.1.456.

Anyone else have these issues?

PS1: Probably irrelevant, but when reporting this I did have another project running which was stored on my hard disk.

PS2: For me, the 2017 problem with opening projects seems to be resolved. My current problem is with the other file related errors.