Same problem here, this is what I have done so far
I am using a RPi2 with a Linksys WUSB6300 connecting to a Linksys WRT1900. System has been working fine for quite a few years with OpenELEC then we started having problems a couple of months ago when I upgraded to LibreELEC. Initially was intermittent connection errors, mostly upon a short cycle of the system. We simply waited a few minutes and connected by hand.
Then I went away for a three day sailing trip and the issues really began. No end of text messages from the family letting me know there were problems! I came back last night, reproduced the error, confirmed all other wifi devices were working fine. Then did the following:
Rebooted the router; same issue.
On the router removed LibreELEC device; same issue.
Restore LibreELEC from a backup; same issue.
Tried a fresh install of LibreELEC; same issue.
Built and OSMC image; same issue. (bloatware! ugh)
Tried an old version of OpenELEC; same issue.
Went back to my original LibreELEC; same issue. (I expected as much wasn't going to try to debug with other systems)
Used my cell phone's hotspot; worked fine.
Went into the Linksys and cleared history of all devices; able to connect to my 5G network!
Unfortunately the problem is not yet solved.
I still have to connect to the network manually when rebooting.
When trying to add my 2.4G network to the system, get the "network error ..." message then system reboots.
I still experience random reboots even on low bandwidth/cpu/memory applications such as listening to streaming radio.