after updating to 188.8.131.52 it's not possible anymore to login on the onepage checkout site.
Only login over header link is possible.
I already checked the request header. FORM_KEY, LOGIN[USERNAME] and LOGIN[PASSWORD] are submitted correct.
If you hit login you get back to the onepage checkout site but without any error message.
And you are still not logged in.
Please back up all your files and overwrite with the files of Default Magento 184.108.40.206
This may fix the issue.