Error when executing action next

System:
Ubuntu 20.04
dataset on a SATA-connected 4TB SSD
nvidia quadro GPU
32GB RAM

Steps to reproduce the problem:
Open phy2, select a unit, hit space

Full error log:

18:22:47.249 [E] qt:178               Timeout in _block().
18:23:24.407 [W] actions:213          Error when executing action Next.
18:23:24.509 [D] actions:214          Traceback (most recent call last):
  File "/home/maxime/anaconda3/envs/phy2/lib/python3.7/site-packages/phy/gui/actions.py", line 211, in wrapped
    return callback(*args)
  File "/home/maxime/anaconda3/envs/phy2/lib/python3.7/site-packages/phylib/utils/event.py", line 141, in emit
    res.append(f(sender, *args, **kwargs))
  File "/home/maxime/anaconda3/envs/phy2/lib/python3.7/site-packages/phy/cluster/supervisor.py", line 847, in _on_action
    _block(lambda: not self._is_busy)
  File "/home/maxime/anaconda3/envs/phy2/lib/python3.7/site-packages/phy/gui/qt.py", line 182, in _block
    raise RuntimeError("Timeout in _block().")
RuntimeError: Timeout in _block().

Reproduce the failure in phy2 by selecting a unit and pressing Space, then inspect phy/cluster/supervisor.py around _on_action, phy/gui/qt.py around _block, and phy/gui/actions.py around the reported wrapper. Trace why the action remains busy until _block() times out; done means the Next action no longer raises the reported RuntimeError under the stated setup.