We’re going to need full debug logs to get any idea of what is going on here.
First make sure you have run updates as there are quite a number of updates post-RC3.
Can you then enable debugging in Kodi, reboot, and try to reproduce your problems (failed addon install) then use the log uploader to upload full logs ? If the log uploader can’t upload for some reason you could use the save to SD card option which will save the log files on the fat32 partition on the SD card so you can retrieve them on a PC.
If you’re seeing a curl timeout it does suggest a connectivity problem to the server hosting the addon - the timeout is 30 seconds by default which means no data was received from the server for 30 seconds, which is a long time…