Instead of appearing one single correlation of the signal of the carbon at 20.9 ppm (#11 in the attached figure), aligned to hydrogen at 2.29 ppm, it appeared two correlations (2,32 and 2,26 ppm). The other nuclei are as expected.
Are you sure that it is an HSQC spectrum that you are showing? I am almost sure that what you show is a HMBC and you are seeing is the coupling to a one bond, from carbon 11 to its same H, which in HMBC is usually shown as a doublet
Yes, it is the HSQC. Indeed it looked like an HMBC 1-bond coupling, which really also appeared in the HMBC experiment, but with a higher coupling constant, as you can see in the figure below (HMBC in blue and HSQC in red).