If I understand correctly,in your write text to a file action , the file path is network location?
Do you use windows OS - if yes make does the agents on the problematic server type running as domain user that can access the network resource?
On Linux OS you should check how the agent daemon is run , and the mount options to the remote resource.
I would also suggest to run the test with single server type and single agent - just to make sure there is no locking problems