A word group that begins with although cannot be linked to a word group that begins with but or however. The result is an error called a mixed construction (see also 11a). Similarly, a word group that begins with because cannot be linked to a word group that begins with so or therefore.

If you want to keep although or because, drop the other linking word.

If you want to keep the other linking word, omit although or because.

For advice about using commas and semicolons with linking words, see 32a and 34b.