Cannot access TimeMachine backups from before clean install of macOS
Time Machine can be tricky and sometimes you really need to reformat your external drive and start over. But you could check several things, like are you using HFS+ on your external Hard Drive? Did you repair permissions of your external hard drive with 'Disk Utility'? Is the internal and external hard drive name still the same? Is your user short name the same?
You could also try, within the information panel of your ext. hard drive, to ignore permissions and user.