cant enable theme in multisite

Finally I’ve found that, in wp-sitemeta DB table, allowedthemes record value was incorrectly serialized (dont know why) and there was a:4 (instead of a:3, because actual length of array was: 3 values of s:)

a:4:{s:13:"twentysixteen";b:1; s:5:"meris";b:1; s:8:"my_theme";b:1;}

I changed a:4 to a:3 and it worked. (also, make sure there is no more allowedthemes rows in wp-sitemeta table)