I've still got an installation of 68.12.0 which also has addon extensions 'ImportExportTools NG ' version 10.0.4 I use a specfic copy of a profile for test/forum help purposes.

So I got it running and ran a couple of tests. Right click on folder > 'ImportExportTools Ng' > Export all messages in the folder > HTML format selected a folder and click on 'Select Folder'. Get all the usual are you sure you want to do this messages and ok it. All messages exported. Check the selected folder to locate a new folder of folder name+date. It contained a 'messages' folder and it contained all the emails in html format. I had 15 emails in folder and there are 15 emails in html format exported.

I tested this on a regular pop account folder and also on imap account folder ( first test I had an mbox file so emails fully downloaded, then a second test after deleting the mbox file so I only had index file of headers.) both had positive correct result.

So this is working for me based on your setup versions.

Suggest as a test: go into 'Offline' mode to ensure there is no synchronisation with folders and see if you can get the export to work. Make sure the Anti-Virus is not able to scan files - the best test to ensure no AV etc in interfering is to restart computer in 'Safe Mode with Networking' mode, start Thunderbird as normal. See what occurs with those two tests to help rule out interference whilst exporting.

I am using this addon ImportExportTools NG https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/