Hello, Fir, and thank you for the report.  I just tested BookMacster's Import and Export with the latest version of Opera (63.0.3368.94) and it worked fine for me.

The error you reported indicates trouble in identifying the “hard folders” in Opera.  This is not too surprising, because, sadly, the Google Chrome bookmarks programming interface which has been adopted by Opera and other browsers, and which we follow to import and export with these browsers, has never stated how to identify the hard folders, and since all of these browsers have slightly different hard folder structures, there is no de facto standard either.  We have therefore reverse-engineered each browser to get this detail.  Possibly Opera has recently introduced some variation into their structure which depends on some variable in the users’ bookmarks.

So we could probably fix this without too much fuss (in less than a week) if you would kindly send us your Opera bookmarks data so we could see what peculiarity triggers this issue.  To do that,

• Run BookMacster, Synknmark Markster or Smarky.
• Click in the menu: Help ▸ Trouble Zipper.
• For Question X (Extra Extension Tests), answer “No".
• For Question B (Bookmarks and Documents), answer "Include Bookmarks and Documents"
• In the list which appears, select “Opera"
• In the "Please explain..." section,  you can just leave it blank because we’ll be looking for you.
• "Dropbox" us the resulting .zip, as instructed.