I am also facing similar issues with many negative frequencies but no COSMO ....just simple ground state optimization within the latest NWChem-6.5
I tried rerunning with 'task dft freq numerical'
There seems to be an oversight in the code. NWChem-6.3 does not have analytical COSMO frequencies. The code should have detected that and switched to numerical frequencies but it does not do that. I have fixed this in the development version now but for the NWChem-6.3 release please use