ERR_CONTENT_DECODING_FAILED – depending on thme

Following situation:

I´ve got
two Domains (A and B) with
two data bases and
two WP installations

Domain, data base and WP installation for “B” is new. I copied all files from A in data base B and changed the prefix here.

When I activate a selfmade theme in B, i get the following error:
ERR_CONTENT_DECODING_FAILED
A works perfekt with this theme. With a default theme B runs perfekt, too.

I had a look at gzip, it´s not enabled. But that´s not the problem, because it´s not enabled for A, too.

Thanks for your help!