I replaced the current implemention with another one (http://
cvs.savannah.gnu.org/viewvc/gnulib/gnulib/lib/base64.c?rev=HEAD), and
encoding result changed from :
AHRlc3QAMGM3ZmJhNTNlNA==
to:
AHRlc3QAMGM3ZmJhNTNlNQ==

And I can login again.

Here is the patch:

diff --git a/ext/libstrophe-0.7.3/src/sasl.c b/ext/libstrophe-0.7.3/
src/sasl.c
index 3f2a8f3..417ae6e 100644
--- a/ext/libstrophe-0.7.3/src/sasl.c
+++ b/ext/libstrophe-0.7.3/src/sasl.c
@@ -393,44 +393,43 @@ char *base64_encode(xmpp_ctx_t *ctx,
     clen = base64_encoded_len(ctx, len);
     cbuf = xmpp_alloc(ctx, clen + 1);
     if (cbuf != NULL) {
-       c = cbuf;
-       /* loop over data, turning every 3 bytes into 4 characters */
-       for (i = 0; i < len - 2; i += 3) {
-           word = buffer[i] << 16 | buffer[i+1] << 8 | buffer[i+2];
-           hextet = (word & 0x00FC0000) >> 18;
-           *c++ = _base64_charmap[hextet];
-           hextet = (word & 0x0003F000) >> 12;
-           *c++ = _base64_charmap[hextet];
-           hextet = (word & 0x00000FC0) >> 6;
-           *c++ = _base64_charmap[hextet];
-           hextet = (word & 0x000003F);
-           *c++ = _base64_charmap[hextet];
-       }
-       /* zero, one or two bytes left */
-       switch (len - i) {
-           case 0:
-               break;
-           case 1:
-               hextet = (buffer[len-1] & 0xFC) >> 2;
-               *c++ = _base64_charmap[hextet];
-               hextet = (buffer[len-1] & 0x02) << 6;
-               *c++ = _base64_charmap[hextet];
-               *c++ = _base64_charmap[64]; /* pad */
-               *c++ = _base64_charmap[64]; /* pad */
-               break;
-           case 2:
-               hextet = (buffer[len-2] & 0xFC) >> 2;
-               *c++ = _base64_charmap[hextet];
-               hextet = ((buffer[len-2] & 0x03) << 4) |
-                        ((buffer[len-1] & 0xF0) >> 4);
-               *c++ = _base64_charmap[hextet];
-               hextet = (buffer[len-1] & 0x0F) << 2;
-               *c++ = _base64_charmap[hextet];
-               *c++ = _base64_charmap[64]; /* pad */
-               break;
-       }
-       /* add a terminal null */
-       *c = '\0';
+
+  int inlen = len;
+  int outlen = clen + 1;
+  const unsigned char* in = buffer;
+  char* out = cbuf;
+  static const char b64str[64] =
+
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+  while (inlen && outlen)
+    {
+      *out++ = b64str[(in[0] >> 2) & 0x3f];
+      if (!--outlen)
+       break;
+      *out++ = b64str[((in[0] << 4)
+                      + (--inlen ? in[1] >> 4 : 0))
+                     & 0x3f];
+      if (!--outlen)
+       break;
+      *out++ =
+       (inlen
+        ? b64str[((in[1] << 2)
+                  + (--inlen ? in[2] >> 6 : 0))
+                 & 0x3f]
+        : '=');
+      if (!--outlen)
+       break;
+      *out++ = inlen ? b64str[in[2] & 0x3f] : '=';
+      if (!--outlen)
+       break;
+      if (inlen)
+       inlen--;
+      if (inlen)
+       in += 3;
+    }
+
+  if (outlen)
+    *out = '\0';
     }