Steam.exe (main exception): CMultiFieldBlob(pserialized): Bad field - extends past end of blob

Option 1: (Official)
https://support.steampowered.com/kb_article.php?ref=3093-MKUJ-5773

Option 2: (Unofficial)
Exit Steam
Navigate to your Steam installation directory. (C:\Program Files\Steam\ by default.)
Delete ClientRegistry.blob