By default, Service version is logged in using the "LocalSystem" account. This account is different to a user account you may be accessing the machine running Syslog with. It does not know about any mapped drives that are defined and are accessible by your user account.
To fix the issue, you may try to consider the following:
1. Give Syslog Service a domain account that has access to share and directory being shared.
-from Windows services.msc, right click on Syslog service>Properties>Log On
2. Give Syslog a UNC path to share (\\server\sharename\dir\), not a map drive (x:\dir). Services don't see map drives.