No metahumans are visible in "my metahuman"

Yes, apparently as mentioned before, the only solution is to open an older MHC character in a newer version and delete it. However, if the invalid character was made in the latest version (currently 5.4), you’re out of luck. In my case, what causes the invalid character is the duplication of the character before sculpting it in MHC. I strongly advise to manually duplicate the character before sculpting and to not use the automatic duplicate feature for now.