Thanks for posting your issue, could you send a save before this happens? For sending save, dump, debug.err files. Would you kindly, submit a ticket via our support system so that I can better assist you with your issue. After following the link, run the support tool, include your steam name, describe the issue you are having and then send us the zip file to greatly speed up the process!
Link to forums for explanation:
https://forums.galciv3.com/452855/page/1/
Link to support tool:
http://sd.stardock.com/Support/supportToolTest/SDSupportTool.exe