When an Agent participates in a job, it creates a hidden service directory .sync in the root of the folder. It's created by each agent in a Sync or Script job, and by destination agents in Consolidation & Distribution job.
The crucial file inside .sync is the identifier of the folder - ID file, by which the agent know that it's the directory that shall be synced. All agents in one job share same identifier (a hash) encoded into ID file.
If anything happens with the ID file, an error is raised by an agent and data delivery stops. There might be several cases when an error about it will appear.
1. Not enough permissions to create identifying .sync/ID file for this shared folder
The error means that agent was not even able to create ID file. Basically, it means that the user that runs the agent may be lacking read-write access in the selected folder. Revise the folder preferences and fix permissions. If this is a mounted folder - revise mount permissions, too.
However, agent won't recover from the error - after fixing permissions remove the agent from the job and add it back again.
2. Share's identifying .sync/ID file is missing
The error means that ID is simply deleted. Unfortunately, the Agent cannot know what might have deleted it and the best fix here is to remove the agent from the job and add it again so that it creates new ID. Also, for Sync jobs, it's possible to copy the ID from any other Agent in the job to avoid reindexing all the files.
3. Share's identifying .sync/ID file is broken
Basically it means that the content of ID file has changed for the agent. The most frequent reason for it is that this folder is used for another job by this agent and thus ID file has new job's identifier recorded there. It's highly advisable not to point the agent into same directory for different job. The fix here is to reorgranize the jobs so that agent syncs different directories with each job. You might want to contact support to get help on how to achieve your setup goals.
It's also possible that the ID file was just corrupt and is now filled with rubbish symbols or zeroes. Remove the agent from the job and add it back again for ID file to be recreated.