Can't open LuCI: Text file busy

Hi, I have a Redmi AC2100 running OpenWrt (snapshot r15241-3ab695368a / LuCI Master git-20.339.75073-e54708a) , and this router has been running for a while now without problem.

But today, when I want to access LuCI, I got this message:

Unable to launch the requested CGI program:
  /www/cgi-bin/luci: Text file busy


And connection to SSH is immediately closed after the connection is established (via PuTTY or SSH-ing from another OpenWrt router).

Rebooting the router (by unplugging and plugging back the power cable) fixed the problem, but any ideas why it happened?