Strange bluish blotch in picture taken with Takumar 50mm 1.4

[wolan]

Hi,
the picture below was taken with the takumar 50mm f1.4.
Do you have an idea where that blue blotch in the middle of the photograph comes from?
The picture has not been touched, just exported from Lightroom with default settings.
I have other pictures taken on the same shooting without any problems.

Thanks.

[Boris_Akunin]

http://thesybersite.com/minolta/sensor-reflection/