This is what frankie said on the phpmotion-V3 support forum:

"for those of you on servers that do not have php running in cgi mode.

open up the htaccess file and look for these lines:

# uncomment this line to enable site wide gzip to increase load times
# may cause 500 server errors on servers running php in cgi mode
#---------------------------------------- GZIP BEGIN ------------------------------------#
# php_value output_handler ob_gzhandler
#---------------------------------------- GZIP END --------------------------------------#

uncomment this line:
php_value output_handler ob_gzhandler

then reupload this increases V3 load times by a huge factor.
if you get the dreaded 500 internal server error then you are kinda out of luck for now, or
at least untill we find a work around.

have fun with V3 !!!
cheers guys

i have notice some of you have tried to do a V2 to the V3 install,
if you have issues, do not report these as bugs, as the site will not be right if you did
this manual upgrade."

So, I ask: does glowhost's servers run our phpmotion sites in cgi mode? Thank you.