Jupyterlab 'waiting for localhost' infinite cycle after upgrade

Jupyterlab was working fine for months until I upgraded today. Now command ‘juptyter lab’ launches a new tab in Firefox, but cycles between ‘waiting for localhost’ and ‘read localhost’. Command ‘jupyter notebook’ continues to work.

Upgraded from:
jupyter-server 1.4.1 to 1.5.1
jupyter-client 6.1.11 to 6.1.12
jupyterlab 3.0.10 to 3.0.12
notebook 6.2.0 to 6.3.0
(and others)

python 3.8.8
miniconda 4.9.2
centOS linux 3.10.0

Tried the following install downgrades, still the same problem each time:
jupyterlab 3.0.12 to 3.0.10
jupyter_client 6.1.12 to 6.1.11
jupyter_server 1.5.1 to 1.4.1
notebook 6.3.0 to 6.2.0

Same problem on a fresh python install on a similar box.

Tried rebuilding jupyterlab (‘jupyterlab build’). Builds OK, no difference.

Tried another from scratch install, python and pip only, no conda. Same version numbers. Same problem.