DJM-REC gives 401 auth error with Dropbox even when you authorise

I took out an expensive Rekordbox Pro sub with a view to switching from iCloud to Dropbox for my music backups and also use Dropbox for DJM-REC to PC transfers, and I am totally regretting it.

I've wasted two months with the terrible dropbox customer support team. Endless backwards and forwards via email (with 3 days delay to every response) forever before they closed the support ticket because I let two days pass without answering yet another extremely technical and confusing set of instructios to try (which involved abandoning DJM-REC to save the files).

I hvae used DJM-REC with iCloud with no problems whatsoever.  Other iPhone apps work equally well with Dropbox, but DJM-REC alone refuses to let me us Dropbox to save files. Every time I hit "Upload" it asks me to give permission for access to Dropbox, which I give and then it fails a few seconds later with a "401 authentication error."  It is ONLY the DJM-Rec app that has this issue.

Dropbox support seem clueless. I've had three escalations up the engineer chain, all wasting time on nonsense like supplying videos of the error (which has to be small enough to fit into an email because they refuse to allow links to even public dropboxes of video 'for security reasons' and say they will close support tickets if you can't comply. Getting video down to a tiny size to pass email is a nightmare!). They've had me uninstallng the app, clearing cache, rebooting, sending endless videos that should not be needed etc.  Nothing they've suggested has fixed the 401 authentication error.

So is anybody successfully using DJM-REC and able to save to Dropbox OK, or have any suggestions as to what might be causing this issue?

I may well have to abandon my Pioneer DJ Pro license as "not fit for purpose" and go back to iCloud and my own manual backups of music if this can't be sorted.  I rip a lot of vinyl and the move from iCloud to Dropbox has completely trashed my ability to do this.