If sinniss’ suggestion works, great. But my personal experience, FWIW, was that I had to loosen the ring as well. And for me, the ring was very tight, so I really needed the c-spanner in order to be able to get enough leverage.
Thanks to all. Looking at a photograph of the 5.2 eccentric bottom bracket on this forum, it seems clear to me that one risks chewing up the soft aluminum housing if the belt is adjusted without loosening the locking ring.
I have found a great source for the 58-62mm C Spanner needed (no - local bike shops are no help, at least here in the Pacific North-West). It is the AMF 54668 hook spanner, in stock and available for $20.50 + postage, from Samstag Sales in the USA. They take PayPal, and can be contacted at manager@samstagsales,com.