phpBB uses encoding as described in RFC 2047 to allow the use ot UTF-8 in email subject lines. However, phpBB uses a single space as a separator between encoded words, whereas the RFC specifies the separator should be CRLF SPACE:

An 'encoded-word' may not be more than 75 characters long, including 'charset', 'encoding', 'encoded-text', and delimiters. If it is desirable to encode more text than will fit in an 'encoded-word' of 75 characters, multiple 'encoded-word's (separated by CRLF SPACE) may be used.

One result of this seems to be that GMail displays a space at the 49th position within subject lines of emails sent out by phpBB boards (obviously, this may split a single word in two). I don't know of any other mail clients that have problems with this.