This is an apt ordering bug. The amd64 version of libsasl2-2 and libsasl2-modules must be unpacked first before trying to unpack the i386 version.
From the attachments to your bug report, it looks like this issue was encountered because you have flashplugin-installer on your system, which gets transitioned to an i386 package between alpha and beta, explaining why new multiarch library packages are being pulled in.
This is an apt ordering bug. The amd64 version of libsasl2-2 and libsasl2-modules must be unpacked first before trying to unpack the i386 version.
From the attachments to your bug report, it looks like this issue was encountered because you have flashplugin- installer on your system, which gets transitioned to an i386 package between alpha and beta, explaining why new multiarch library packages are being pulled in.