Rails - Question mark inserted into database

Question

I noticed a strange behaviour of an application today which I can't explain:

We have an email queue to send mails via ArMailer. In some of them a question mark is inserted from nowhere.

This is from the insert command rails shows me in the server log:

Outstanding: &euro;999.00<br /><br />

What I have in the database afterwards is:

Outstanding: &euro;999.00?<br /><br />

Where could this question mark come from? It happens reproducible only at this position.

Thanks in advance!

Edit:

This does not happen to other similar parts of the email, e.g.

Deposit: &euro;114.00<br /><br />

is written into the database correctly.