Short answer: Copy-paste the HTML code rather than the rendered signature
A client of mine created a brand new signature combining images, text and links, in HTML format.
Blog (EN)
Short answer: Find out what specific part of the boot sequence fails and, if needed, 'google' the problem
The first thing to try would be rebooting in "Safe Mode" which cleans some cache and only loads the minimum from the system to operate.
If that doesn't work, then you'll need to reboot the computer in "Verbose Mode", see where the login procedure is stuck and Google the problem
Did you know that it's possible to tell Finder to search the current folder rather than the entire file system?
By default, regardless of the folder you're currently in, using the Search bar (top right corner) will show results across the entire file system (aka This Mac).
To look for results in the current folder, you then need to click on its name (i.e. "Newsflash")