Error in kopano-backup

KaBi

Dear forum,
we are running kopano-backup with a cron-job, it was running fine until 2 days ago. The following error occurs, does anyone know what the reason could be?
Thanks in advance for your answer.

2021-05-13 22:10:41,824 - backup0 - ERROR - Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/kopano/log.py", line 103, in log_exc
    try: yield
  File "/usr/lib/python3/dist-packages/kopano_backup/__init__.py", line 220, in main
    self.backup_hierarchy(path, stats, options, store, user, server, config)
  File "/usr/lib/python3/dist-packages/kopano_backup/__init__.py", line 257, in backup_hierarchy
    folders = list(store.folders())
  File "/usr/lib/python3/dist-packages/kopano/store.py", line 423, in folders
    for folder in self.subtree.folders(recurse=recurse, **kwargs):
  File "/usr/lib/python3/dist-packages/kopano/folder.py", line 720, in folders
    for f in folders_recursive(rootfolders):
  File "/usr/lib/python3/dist-packages/kopano/folder.py", line 710, in folders_recursive
    for feid, f in sorted(fs, key=lambda data: names[data[0]]):
TypeError: unorderable types: int() < str()