TL-WR841ND V8: Low memory, then no web interface

Hello everyone,

After installing a luci DDNS package, we can't login into the web interface anymore:

/usr/lib/lua/luci/controller/ddns.lua:18: module 'luci.tools.ddns' not found:
	no field package.preload['luci.tools.ddns']
	no file './luci/tools/ddns.lua'
	no file '/usr/share/lua/luci/tools/ddns.lua'
	no file '/usr/share/lua/luci/tools/ddns/init.lua'
	no file '/usr/lib/lua/luci/tools/ddns.lua'
	no file '/usr/lib/lua/luci/tools/ddns/init.lua'
	no file './luci/tools/ddns.so'
	no file '/usr/lib/lua/luci/tools/ddns.so'
	no file '/usr/lib/lua/loadall.so'
	no file './luci.so'
	no file '/usr/lib/lua/luci.so'
	no file '/usr/lib/lua/loadall.so'
stack traceback:
	[C]: in function 'require'
	/usr/lib/lua/luci/controller/ddns.lua:18: in main chunk
	[C]: in function 'require'
	/usr/lib/lua/luci/dispatcher.lua:568: in function 'createindex'
	/usr/lib/lua/luci/dispatcher.lua:595: in function 'createtree'
	/usr/lib/lua/luci/dispatcher.lua:262: in function 'dispatch'
	/usr/lib/lua/luci/dispatcher.lua:127: in function </usr/lib/lua/luci/dispatcher.lua:126>

How can we free some memory? Tried to delete DDS package related folders and files using putty, but we get a read-only file system error when trying to delete ddns.lua.