I installed the comp. today and was having the same problem. But you can save it without having to do anything to the files.
Open the config and chagne your settings, then click on save. Now, when it gives you the popup about the purge, click on cancel and it will save the config file.
I tried it and it works. Each time I clicked on Ok, nothing happened, then I just happen to click cancle instead of Ok at the purge request and it worked.
May be a coding error or something, but that is how to do it.