401 When posting images with HTML in caption text

[hostj2me]

My Android application posts images from with captions. For certain
captions, the application is receiving a 401 error. The error is non
predictable: using the same exact text from a Tumblr post as a caption
can cause the error.

Are there any special HTML tags or sequences that cannot be contained
in a caption? Are the spam checks or filtering mechanisms that may
also cause the error?

Thank you.

[hostj2me]

Here is a example of a post that fails with a 401

http://modernlifestyle.tumblr.com/post/19791668129/micasaessucasa-x-house-by-arquitectura-x

I can copy the post text byte for byte, use it as text in a new post,
and the post will fail with a 401.

If I remove the text (caption), still using the same image, the post
is successful.

Here is an example of a post with a caption that can be copied byte
for byte and works:

http://modernlifestyle.tumblr.com/post/19791862540/1-billion-real-madrid-resort-island-planned-for