You could enable Show custom thread names in htop display options, then it should show what the thread does that uses that much CPU time.
Best tool for Linux recently that finds memory leaks/usage problems is heaptrack.
- Compile the urbackup server (so you have debug symbols)
- Compile or install heaptrack https://github.com/KDE/heaptrack
- Then run urbackupsrv with e.g.
heaptrack urbackupsrv run -c /etc/default/urbackupsrv - Send me the heaptrack output or analyze it yourself