yes you're partly correct Timm, rounded cornors is an issue with early danish issues but only those with KI or line perforation.
yours is a KI perforation, however yours is defective because the missing cornor is not due to "seperating the stamps" the SV cornor is also missing one perf. and then perf 3+4 on the south (these are likely to be from seperation)
perforation varies alot on KI, the attatched picture shows an example of how a stamp from 9th and 10th row could be born without the SE cornor perf. Notice this is not due to seperation.
there are also examples of stamps with a cornor perf that is so weak it would almost certainly break off at some point.

even in denmark most do not know this, and this stamp would be considdered defective (which it also is) and the desirability would suffer along with the price.
your stamp would cost between $0,5 and 1,5 in Denmark, even a stamp which clearly is just a perforation issue wouldn't cost more, because most lack the insight.