I couldnt find them anywhere. Try cooker/cooker, root/toor and admin/admin? Otherwise you have this method to reset the password Reset root or user password - #3
Of course I assume you know what you’re doing if you chose a server install. .img images are supposed to be flashed directly to the desired disk via the ‘dd if file.iso of=/dev/sdX’ command. It is not meant to be flashed on an usb drive as it is already a fully installed system. If you want installer images, these are the .iso files, thatb can be flashed to usb. However I don’t think openmandriva has one of these for a server edition, maybe try the minimal edition (most similar one to a server install, but there are currently no build successful): Product minimal cooker x86_64 - Platform cooker - Openmandriva ABF
Or just take a very light desktop and uninstall it afterwards or keep it. Its honestly not too much of a CPU hog these days, but I understand wanting to minimize package numbers for security: Product cooker plasma6.x11 slim x86_64 - Platform - Openmandriva ABF