I'm running Windows Server 2012 R2 / IIS 8.5 and this has been running fine for ages. Updated to the 7.4 2015-06-18 beta and then hit my site with a Win10 IE11 and Edge browser.
This started causing AWStats to stop processing the log files and throw an error that my log formatting wasn't correct. As soon as I removed the Win10 log entries from the log file, the logs started processing again.
Sample logfile entry causing issue:
2015-07-07 14:27:03 GET /Cats/cats.html - - 124.169.2.198 HTTP/1.1 Mozilla/5.0+(Windows+NT+10.0;+Win64;+x64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/42.0.2311.135+Safari/537.36+Edge/12.10164 http://anthonyb.no-ip.biz/ 200 844 31
Please let me know if there is any other data you need!