Thanks everyone! Well, I managed to open it and I've washed my top and my shirt.

I didn't even have to put it in hot water, but I had to use a bit of force to open it and it slipped, so a bit of it went on my clothes and the table.

I made sure to clean the edge and the rubber band thoroughly before I closed it again - and I rubbed a tiny bit of oil on the rubber ring. So, next time it should open without problems.