RSAKey construction is incorrect. In current implementation problem exists only for RSAKey construction from istream (just luck).

Reason is - using BIO_seek() instead of BIO_reset() between PEM_read_bio_RSAPublicKey() and PEM_read_bio_RSA_PUBKEY()
This works for construction from file (File BIO), but incorrect for stream (Memory BIO)

Here is patch:

109c109
<               int rc = BIO_seek(bio, 0);

---
>               int rc = BIO_reset(bio);
170,171c170,171
<           int rc = BIO_seek(bio, 0);
<           if (rc != 0) throw Poco::FileException("Failed to load public key");

---
>           int rc = BIO_reset(bio);
>           if (rc != 1) throw Poco::FileException("Failed to load public key");

Affected versions 1.4.6p2 - 1.4.6.p4 may be other.