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The maitenance work done on the server could (theoretically, as I don't know) have overwritten the customised and optimised db settings with default ones. Or a temp folder could have been full.
As XOOPS.org is hosted on 2 dedicated servers with enough bandwidth to spare, that shouldn't have been the problem (but again, I don't know for sure).
My vote is on misconfiguration because of the server upgrade work. And that could be a thousand things, so it's not that difficult to miss.
Herko