That can happen if you leave the PM-compose page open for a long time so that your session-code becomes invalid. Reloading the page should fix it.
(The JS autocompleter function should detect this and not dump the whole HTML error page, but it's a rare situation, and it's not immediately obvious to me how to fix it.)
I think this is what happened though I can't really remember vividly since I was on my mobile device while this issue occurred. Nevertheless thanks for noting it out . I'm not much of a JavaScript coder else probably I would have looked up a way to fix it.
Alternatively he can still use the find Members button beside the "To" column. This will pop up a search bar where he can search for members by their username, if found he/she will simply tap on it and it will be added to the "To" column then when he/she has gotten the exact number of Members he/she wants to send the pm to then moving the names to the "BCC" column will be the next step that way the name accuracy will be 100% and it eliminate errors as well.
Yeah you do have a point, this too is feasible but I don't know if you've tried it before but if you have you would notice that sometimes even when you spell out the usernames correctly including being case sensitive, it still says user not found.