De-installing Steam, rebooting PC, reinstalling Steam (=> Updated fine, but error messages keep popping up) Deleting all files in the Steam folder, but Steam.exe + Backup of SteamApps (=> Updated fine, but error messages keep popping up) Resetting both internet connection and PC (=> Changed nothing) Logging in to (=> Yup, changed nothing, though)
Checking if other PCs in the network can connect to Steam (=> Working like a charm) PLEASE READ THROUGH THE ENTIRE THREAD, IF YOU HAPPEN TO ENCOUNTER A SIMILAR PROBLEM, I'M SURE SOME METHOD MENTIONED WILL WORK ON YOU :)
EDIT after the issue has been solved, now:
