Try a factory reset of the router to see if that resolves the lighttpd errors. Sometimes updates break software like this if files are left behind that aren't compatible with newer versions of the lighttpd binary.
For the 5Ghz issue, it's possible the router switched to a DFS channel, and your Roku does not support those 5Ghz channels. Some devices do not since the manufacturer opted not to pay for, or implement DFS/TPC certification and support.
This website is a helpful resource for checking to see what devices support DFS. DFS Channels are Channels 52 through 144. Some devices also do not support Channel 165, which can only be used if your router is in 20Mhz mode, which it is most likely not. https://sites.google.com/a/mikealbano.com/clients/home
Alternatively, you may want to try toggling the "Self Organizing Network" feature off or on if the problem is not with DFS channels.