Oil leak?

not likely. Its probably a problem directly related to the oil filter as mharriso suggested, the filter is probably either lose or damaged or hasnt sealed properly. if tightening dosent fix it, then you will have to fit a new filter, and change the oil while your at it.

The oil filter was secured well. I dried the sump. I see that the oil starts too leak at the end of the sump and the filter is starting to get wet slowly (see picture). What can be the source of this sweat/leak?

yeah the filter may well be secured tightly but if the seal is kinked malformed or crushed it will still weep

make sure when you refit a new filter the oil filter rubber seal has been oiled on both faces, this stops the seal binding (lifting and allowing a leak) on either the filter body or the sump face, also do not overtighten as the seal will loose its ability to defrom and seal (it reaches its elastic limit).