A website shows the following error message:
SQLSTATE[HY000]: General error: 126 Incorrect key file for table '/tmp/#sql_1898_0.MYI'; try to repair it
So, MySQL: which table is to repair? The error message is not a table, it's just the temp file.
Stupid and useless error message ...
If you run MySQL and this error occurs:
SQLSTATE[HY000]: General error: 3 Error writing file '/tmp/MYh0VgDS' (Errcode: 28)
then check if your server isn't using the famous "overflow" filesystem for /tmp, limited to 1 MB space.
The MySQL error message is - as usual - misleading and non-descriptive.
There are many other possible causes for this error code, so this one here might just be a hint.