Jamie and I have been 'collaborating' (not the mod) ... he uploaded an .mbz to a server outside of Moodle using Webmin.
That allowed me to inspect what was actually in the backup by un-compressing it and inspecting what was there.
In the un-compressed backup no user backup uploaded, in activities directory
assign_1302 assign_8608 label_1295 lesson_14329 quiz_1298 quiz_1311 quiz_8607 resource_1293 resource_8603
assign_8589 assign_9184 label_9194 lesson_14330 quiz_1300 quiz_8587 quiz_8609 resource_1303 resource_9181
assign_8591 bigbluebuttonbn_8586 label_9216 lesson_14331 quiz_1304 quiz_8588 quiz_8610 resource_14328 url_1294
assign_8593 collaborate_2735 label_9217 page_8613 quiz_1305 quiz_8595 quiz_8614 resource_8581 url_9190
assign_8596 collaborate_9493 label_9221 quiz_1296 quiz_1306 quiz_8599 quiz_9182 resource_8583
assign_8601 forum_9220 lesson_14327 quiz_1297 quiz_1309 quiz_8604 quiz_9183 resource_8598
All those but 'bigbluebuttonbn_8586', 'collaborate_2735 and collaborate_9493 are stock moodle. The numbers are the ID numbers of the assignment/label/quiz, etc..
and in the moodle_backup.xml which is a roadmap to restore:
[root@sos sos]# fgrep 'collaborate' moodle_backup.xml
Can see a 'userinfo' thang ... which I have no idea what it does ... but could have been the source for the user roles conflict.
Course was also fairly 'heavy processing' in quiz:
-rw-r--r--. 1 root root 1234304 Jun 28 18:30 questions.xml
Sometimes courses like this might look small when viewing the size of the .mbz file (only 7723823) but they are heavy processing on restore ... 1.2 megs worth of question data (approx) in quizzes.
One could edit xml files, re-package the backup and then TIA.
All the above NOT the recommended way to fix but when all else has failed it is possible to recoup some of the course.
'spirit of sharing', Ken