I am happy to report that our very large course (abt. 200 multiple-choice quizzes with 2,000+ question in the question bank) will now restore in Moodle 2.7.1.
Previously, it hung at 99.9% finished and never installed the (absolutely essential) gradebook categories. This problem made any Moodle higher than 2.5.6 unusable to us; I was beginning to fear that we could never leave 2.5.5 or 2.5.6. But the new 2.7.1 seems to restore our large course OK. At least it does so today!
There was one adventure, however. Our course has some 50 quizzes with conditions -- e.g., to take quiz 4, you have to complete quizzes 1, 2, and 3 with a grade of 90.
When restoring a 2.5.5 backup into 2.7.1, I found that these conditions were duplicated. Instead of the conditional stating that the student must complete 1, 2, and 3 in order to take quiz 4, the conditions stated that the student must complete quizzes 1, 2, 3, 1, 2, 3, 1, 2, 3. Nearly every conditional quiz had doubled or tripled its conditions in this manner.
(Previously, when I had tried to restore into Moodle versions between 2.5.7 and 2.7.1, the restored version attached multiple new empty conditions to random questions -- conditions that said the current quiz would not be available until the student had completed "missing." These empty conditions were attached to about four dozen quizzes.)
I don't know if this problem with conditions is widespread enough to warrant fixing. Here's how I (finally, I hope) got around it.
Working on the restored course in 2.7.1, I repaired all the conditional quizzes by hand -- deleting the duplicate conditions. This took a while.
Then I backed up that course in 2.7.1 and restored it in 2.7.1. It restored and all parts of the course looked OK.
I then turned on the Moodle new backup format and again backed up and restored. Same good result: Everything looks OK.
I am finally optimistic that we will be able to backup and restore a large course in 2.7.1. (I had been getting worried.)
For anyone who worked to fix that restore problem, thanks for your help! For anyone still struggling with it, Hang in there!
Specs: Running Moodle 2.5.5 and 2.5.6, with the intention of moving to 2.7.1 when we validate that everything works. Quad-core-3.2G, 8G RAM, Red Hat 9.8.2. SSD Linux server.