I was wondering if maybe, since you have multiple profiles, Chrome was requiring that in order to export to Profile X, you must have at least one Chrome browser window open in Profile X. But, to my surprise, that is not the case. Provided that I have a Chrome browser window open in any profile, I can export from BookMacster to all profiles.
If I have zero browser windows open, then exporting to any profile fails, indicating that the Chromium team has shown no interest in fixing this after several years

So then I set up a test of your configuration, 3 profiles in Chrome and 2 profiles in Comet, added all 5 profiles as Clients in BookMacster, then did an
Export to All several times and it worked fine. I have a typical Collection of 427 folders and 1325 bookmarks.
What is the failure rate? I mean, how many times would I need to try an export before likely seeing an error 651507?
I suppose you could send us your data and I could try to reproduce the problem with your Collection. To do that,
• Run BookMacster.
• 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, choose "Google Chrome", and "Comet" if you see it in the menu. I forgot if we've added support for Comet in Trouble Zipper yet. But Trouble Zipper only picks up the first profile anyhow. The important thing is that I get your BookMacster Collection, which is a "Document" so it should be included.
• In the "Please explain..." section, leave it blank unless you have more information to tell.
• Send me the resulting .zip via MailDrop, Dropbox or whatever, as instructed.