Can no longer re-export to .png

[retrogrouch9]

I LOVE this project! Using the web version via Firefox, I created several panels a couple of days ago, repeatedly saving the working files (.htck) and repeatedly exporting the panels as .png files to view them on my Debian Buster system using ImageViewer. Then I would go back to the application, load a panel, tweak it, resave the .htck and re-export the .png. Rinse and repeat. Several times. No problems.

Two days later, viewing the panels again, I decided two needed minor changes. I invoked the web version, loaded the respective .htck file, made the tweaks, saved the updated .htck file, clicked the button to export as .png, that button changed to a "waiting" sort of animation, and the dialog box for saving the file NEVER appears. The "waiting" animation goes on until I close the application.

I also tried loading .htck files (worked) and exporting as .png (didn't work, same problem) in Chromium and Gnome-Web browsers on an up-to-date Debian Buster system.

More troubeshooting results: I can create a new panel, drag some figures and text onto it, save the .htck, and export as .png, no problem. I can load any of the .htck files I created two days ago, but I can no longer export ANY as .png.

HELP PLEASE. I need to get these panels updated and printed asap.

I tried to attach two of the .htck files for your inspection, but Github tells me it does not support that file type. OK. I took a screenshot and tried to attach the screenshot.png: Github says "Something went really wrong, and we can't procees that file. Try again." OK, I zipped two of the .htck files and tried to attach the resulting htckfiles.zip, same "really wrong" message. Sorry, no luck.