The myDBR root directory has temporary files, named phpxltmp*.
Inspecting these show that these are likely xlsx files.
There are not many, and I think they are from a timed out / out of memory / other error condition while generating a xlsx from a report.
1. Can you put temporary files where they belong (in sys_get_temp_dir())
2. Can you try and clean up the temporary files even when a crash occurs?