I’m encountering the same error in multiple attempts with multiple images in both Firefox and Chrome (WP v5.5, Regenerate Thumbnails v3.1.3). FFox browser console showed no errors or warnings throughout the attempted regeneration process; here’s the Chrome console output:

/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839:49 Mixed Content: The page at 'https://engineering.berkeley.edu/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839' was loaded over HTTPS, but requested an insecure stylesheet 'http://engineering.berkeley.edu/wp-content/plugins/publishpress-pro/vendor/alledia/wordpress-plugin-framework/src/assets/lib/font-awesome-v5.2.0/css/all.min.css?ver=0.6.8'. This request has been blocked; the content must be served over HTTPS.
/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839:50 Mixed Content: The page at 'https://engineering.berkeley.edu/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839' was loaded over HTTPS, but requested an insecure stylesheet 'http://engineering.berkeley.edu/wp-content/plugins/publishpress-pro/vendor/alledia/wordpress-plugin-framework/src/assets/css/allex-grid.min.css?ver=0.6.8'. This request has been blocked; the content must be served over HTTPS.
/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839:51 Mixed Content: The page at 'https://engineering.berkeley.edu/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839' was loaded over HTTPS, but requested an insecure stylesheet 'http://engineering.berkeley.edu/wp-content/plugins/publishpress-pro/vendor/alledia/wordpress-plugin-framework/src/assets/css/allex-admin.css?ver=0.6.8'. This request has been blocked; the content must be served over HTTPS.
load-scripts.php?c=0&load[chunk_0]=hoverIntent,common,hoverintent-js,admin-bar,svg-painter,thickbox,jquery-ui-draggable,jquery-ui-button,jquery-ui-position,jquery-&load[chunk_1]=ui-dialog,wp-api-request&ver=5.5:4 Uncaught ReferenceError: wp is not defined
    at load-scripts.php?c=0&load[chunk_0]=hoverIntent,common,hoverintent-js,admin-bar,svg-painter,thickbox,jquery-ui-draggable,jquery-ui-button,jquery-ui-position,jquery-&load[chunk_1]=ui-dialog,wp-api-request&ver=5.5:4
    at load-scripts.php?c=0&load[chunk_0]=hoverIntent,common,hoverintent-js,admin-bar,svg-painter,thickbox,jquery-ui-draggable,jquery-ui-button,jquery-ui-position,jquery-&load[chunk_1]=ui-dialog,wp-api-request&ver=5.5:4
(anonymous) @ load-scripts.php?c=0&load[chunk_0]=hoverIntent,common,hoverintent-js,admin-bar,svg-painter,thickbox,jquery-ui-draggable,jquery-ui-button,jquery-ui-position,jquery-&load[chunk_1]=ui-dialog,wp-api-request&ver=5.5:4
(anonymous) @ load-scripts.php?c=0&load[chunk_0]=hoverIntent,common,hoverintent-js,admin-bar,svg-painter,thickbox,jquery-ui-draggable,jquery-ui-button,jquery-ui-position,jquery-&load[chunk_1]=ui-dialog,wp-api-request&ver=5.5:4
build.js?ver=3.1.3:2 TypeError: wp.apiRequest is not a function
    at a.created (build.js?ver=3.1.3:2)
    at Ve (build.js?ver=3.1.3:2)
    at tn (build.js?ver=3.1.3:2)
    at a.e._init (build.js?ver=3.1.3:2)
    at new a (build.js?ver=3.1.3:2)
    at build.js?ver=3.1.3:2
    at init (build.js?ver=3.1.3:2)
    at n (build.js?ver=3.1.3:2)
    at build.js?ver=3.1.3:2
    at f (build.js?ver=3.1.3:2)
We @ build.js?ver=3.1.3:2
Je @ build.js?ver=3.1.3:2
qe @ build.js?ver=3.1.3:2
Ve @ build.js?ver=3.1.3:2
tn @ build.js?ver=3.1.3:2
e._init @ build.js?ver=3.1.3:2
a @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
init @ build.js?ver=3.1.3:2
n @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
f @ build.js?ver=3.1.3:2
h @ build.js?ver=3.1.3:2
f @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
e._update @ build.js?ver=3.1.3:2
r @ build.js?ver=3.1.3:2
hn.get @ build.js?ver=3.1.3:2
hn @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
$n.$mount @ build.js?ver=3.1.3:2
$n.$mount @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
n @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
(anonymous) @ build.js?ver=3.1.3:2
quick-add.js?ver=3.3.4:1 Uncaught TypeError: t.dialog is not a function
    at HTMLDocument.<anonymous> (quick-add.js?ver=3.3.4:1)
    at i (load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2)
    at Object.fireWith [as resolveWith] (load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2)
    at Function.ready (load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2)
    at HTMLDocument.J (load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2)
(anonymous) @ quick-add.js?ver=3.3.4:1
i @ load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2
fireWith @ load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2
ready @ load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2
J @ load-scripts.php?c=0&load[chunk_0]=jquery,jquery-ui-core,utils&ver=5.5:2
tools.php?page=regenerate-thumbnails:1 Mixed Content: The page at 'https://engineering.berkeley.edu/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839' was loaded over HTTPS, but requested an insecure favicon 'http://engineering.berkeley.edu/wp-content/uploads/2020/02/cropped-En_Favicon_Gold-32x32.png'. This request has been blocked; the content must be served over HTTPS.
tools.php?page=regenerate-thumbnails:1 Mixed Content: The page at 'https://engineering.berkeley.edu/wp-admin/tools.php?page=regenerate-thumbnails#/regenerate/20839' was loaded over HTTPS, but requested an insecure favicon 'http://engineering.berkeley.edu/wp-content/uploads/2020/02/cropped-En_Favicon_Gold-192x192.png'. This request has been blocked; the content must be served over HTTPS.

• This reply was modified 8 months, 3 weeks ago by Yui.
• This reply was modified 8 months, 3 weeks ago by Yui. Reason: please use CODE button for proper formatting

Thanks, @renathoc. I had spotted and fixed that mixed-content error a couple of weeks ago in relation to a different problem. Regenerating thumbnails now is working properly again, so looks like that was the culprit there as well.