Well, it took almost 2 weeks and hundreds of successful uploads, but the problem finally manifested again. This time, a second upload attempt was immediately successful. The last two lines in the developer’s console log seem to point clearly to network connectivity as the culprit:
[Warning] [PLUGIN discourse-adplugin] (vendor-3525a44c5f1c281745e16fd0c569078e0ad263d863a3e3d95fe3a8ca080d252b.br.js, line 4586)
"Deprecation notice: \"message-bus:main\" is deprecated, use \"service:message-bus\" instead (deprecated since Discourse 2.9.0.beta7) (removal in Discourse 3.0.0)"
[Warning] [PLUGIN discourse-cakeday] (vendor-3525a44c5f1c281745e16fd0c569078e0ad263d863a3e3d95fe3a8ca080d252b.br.js, line 4586)
"Deprecation notice: \"site-settings:main\" is deprecated, use \"service:site-settings\" instead (deprecated since Discourse 2.9.0.beta7) (removal in Discourse 3.0.0)"
[Warning] [PLUGIN discourse-canned-replies] (vendor-3525a44c5f1c281745e16fd0c569078e0ad263d863a3e3d95fe3a8ca080d252b.br.js, line 4586)
"Deprecation notice: \"current-user:main\" is deprecated, use \"service:current-user\" instead (deprecated since Discourse 2.9.0.beta7) (removal in Discourse 3.0.0)"
[Warning] [PLUGIN hosted-site] (vendor-3525a44c5f1c281745e16fd0c569078e0ad263d863a3e3d95fe3a8ca080d252b.br.js, line 4586)
"Deprecation notice: \"site:main\" is deprecated, use \"service:site\" instead (deprecated since Discourse 2.9.0.beta7) (removal in Discourse 3.0.0)"
[Error] Failed to load resource: The network connection was lost. (poll, line 0)
[Error] Failed to load resource: The Internet connection appears to be offline. (poll, line 0)
Unless this problem repeats with different symptoms, I think we should consider it resolved. I am puzzled, though, that the console log did not display similar errors following problems after the first fix.