Were you previously running the Certify background service under a user other than Local System?
By default the background service runs as Local System, this means it doesn’t have any permission for network access (such as file copying or remote scripting). This is the only supported configuration.
To get your script to you should be able to edit the Deployment Task (Scripting moved to Deployment > Deployment Tasks in the most recent version and will be moving to it’s own Tasks tab in the next release). The deployment task has parameters for things like saved network credentials.
Assuming you have a working certificate, try a simpler script that just copies the file to see if that works for you (you could alternatively try the CCS Export script which basically does the same thing). As an aside you could also investigate using the Central Certificate Store method of sourcing your certificate for Exchange.
On another thread a user had trouble getting export to work to another machine on their domain and this was fixed by changing our LogonType that we use when impersonating network users. Perhaps you could try the same fix? Export certificate to network/UNC path failed
As an aside, please don’t cross-post your issue in several places, this just bumps conversation threads and doesn’t result in more assistance being offered.