First I have to say: Sorry for the noise!

I found a copy & paste-error in the SQL statement:

u.`name`=plu.`name`=MID(plu.`name`,3,LENGTH(plu.`name`)-4) 

Should be:

u.`name`=MID(plu.`name`,3,LENGTH(plu.`name`)-4) 

The produced error was just misleading in that case.