Issue after reinstall: Couldn't load plugin 'processing'

I encountered an error after reinstalling QGIS (long-term release: 3.16.4-Hannover). I was re-installing using the Debian/Ubuntu instructions from here, but errors started after running sudo apt install qgis qgis-plugin-grass

...
Setting up libqgis-gui3.16.4 (1:3.16.4+32focal) ...
Setting up qgis-providers (1:3.16.4+32focal) ...
    - 'VirtualXPath'    [XML Path Language - XPath]
    - 'VirtualXPath'    [XML Path Language - XPath]
    - 'VirtualXPath'    [XML Path Language - XPath]
free(): invalid pointer
Aborted (core dumped)
dpkg: error processing package qgis-providers (--configure):
 installed qgis-providers package post-installation script subprocess returned error exit status 134
Setting up libqt53dextras5:amd64 (5.12.8+dfsg-0ubuntu1) ...
Setting up python3-pyqt5.qtsql (5.14.1+dfsg-3build1) ...
dpkg: dependency problems prevent configuration of qgis:
 qgis depends on qgis-providers (= 1:3.16.4+32focal); however:
  Package qgis-providers is not configured yet.

dpkg: error processing package qgis (--configure):
 dependency problems - leaving unconfigured
Setting up libqgis-customwidgets (1:3.16.4+32focal) ...
No apport report written because the error message indicates its a followup error from a previous fai
lure.
     dpkg: dependency problems prevent configuration of qgis-plugin-grass:
 qgis-plugin-grass depends on qgis (= 1:3.16.4+32focal); however:
  Package qgis is not configured yet.

dpkg: error processing package qgis-plugin-grass (--configure):
 dependency problems - leaving unconfigured
No apport report written because the error message indicates its a followup error from a previous fai
lure.
     Setting up qt3d-gltfsceneio-plugin:amd64 (5.12.8+dfsg-0ubuntu1) ...
dpkg: dependency problems prevent configuration of python3-qgis:
 python3-qgis depends on qgis-providers (= 1:3.16.4+32focal); however:
  Package qgis-providers is not configured yet.

dpkg: error processing package python3-qgis (--configure):
 dependency problems - leaving unconfigured
No apport report written because MaxReports is reached already
                                                              Setting up libqgisgrass7-3.16.4 (1:3.16
.4+32focal) ...
Setting up qt3d-assimpsceneimport-plugin:amd64 (5.12.8+dfsg-0ubuntu1) ...
Setting up python3-qgis-common (1:3.16.4+32focal) ...
Setting up python3-pyqt5.qtwebkit (5.14.1+dfsg-3build1) ...
Setting up qgis-provider-grass (1:3.16.4+32focal) ...
Setting up python3-pyqt5.qsci (2.11.2+dfsg-6) ...
Setting up libqgis-3d3.16.4 (1:3.16.4+32focal) ...
Setting up libqgis-app3.16.4 (1:3.16.4+32focal) ...
Processing triggers for mime-support (3.64ubuntu1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Processing triggers for gnome-menus (3.36.0-1ubuntu1) ...
Processing triggers for libc-bin (2.31-0ubuntu9.2) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for shared-mime-info (1.15-1) ...
Processing triggers for desktop-file-utils (0.24-1ubuntu3) ...
Errors were encountered while processing:
 qgis-providers
 qgis
 qgis-plugin-grass
 python3-qgis
E: Sub-process /usr/bin/dpkg returned an error code (1)

QGIS is able to run, but displays this error just after running the program:

Couldn't load plugin 'processing' 

TypeError: ' 
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/qgis/utils.py", line 312, in loadPlugin
    __import__(packageName)
  File "/usr/lib/python3/dist-packages/qgis/utils.py", line 792, in _import
    mod = _builtin_import(name, globals, locals, fromlist, level)
  File "/usr/share/qgis/python/plugins/processing/__init__.py", line 35, in 
    from processing.tools.raster import *  # NOQA
  File "/usr/lib/python3/dist-packages/qgis/utils.py", line 792, in _import
    mod = _builtin_import(name, globals, locals, fromlist, level)
  File "/usr/share/qgis/python/plugins/processing/tools/raster.py", line 26, in 
    from osgeo import gdal
  File "/usr/lib/python3/dist-packages/qgis/utils.py", line 792, in _import
    mod = _builtin_import(name, globals, locals, fromlist, level)
  File "/usr/lib/python3/dist-packages/osgeo/__init__.py", line 113, in 
    if python_version TypeError: '

Python version: 3.8.5 (default, Jan 27 2021, 15:41:15) [GCC 9.3.0] 
QGIS version: 3.16.4-Hannover Hannover, 654e76b3cb 

Python Path:
/usr/share/qgis/python
/home/username/.local/share/QGIS/QGIS3/profiles/default/python
/home/username/.local/share/QGIS/QGIS3/profiles/default/python/plugins
/usr/share/qgis/python/plugins
/usr/lib/python38.zip
/usr/lib/python3.8
/usr/lib/python3.8/lib-dynload
/usr/local/lib/python3.8/dist-packages
/usr/lib/python3/dist-packages
/home/username/.local/share/QGIS/QGIS3/profiles/default/python

I found a similar problem here, here, and here, but the solutions here are all Windows-specific (I’m running Ubuntu 20.04). As in the last issue, the program runs, but the Vector menu doesn’t open.

I’ve tried removing and purging, then re-installing newer releases (3.18-Zürich) and the same problem occurs. I suspect it has to do with leftover material from the previous install, but I don’t know how to fix this problem or interpret the error information that QGIS provides.

 # remove offending lib while keeping package dependency installed (wow, ugly):
 rm -f /lib/x86_64-linux-gnu/libproj.so.15
 ln -s /lib/x86_64-linux-gnu/libproj.so.19 /lib/x86_64-linux-gnu/libproj.so.15