More information on the contents of ImpressCMS 1.3.8 are available here on assembla
I'll wait for some more test results before I turn this into a proper news report. Final release, if all goes well, should be 23 june 2015.
The release can be found on Sourceforge : impresscms_1.3.8_beta.zip
In case of bugs, please create a ticket on assembla, or better: fix the problem on assembla (starting from the /branches/impresscms_1.3 branch) and do a merge request.