Kamailio uses its own memory manager, so even you have a lot of memory in that system, you have to instruct Kamailio how much to use via `-m` and `-M` parameters. In this case `-m` is important, be sure you have at least 256 (if you have a lot of tls connections, then make it even higher).
If `-m` value was already higher or happens again later after making it higher, reopen.