The folders are on the agents not on the server. Our agents have the OS Windows.
The content in the folders are log files from the build jobs running on the agent (current more than 30GB).
There is one log file for each job.
The log files are there since we updated Bamboo from 5.10 to 6.1.1 in September 2017. Now we running 6.2.2 and have the still the problem.