20mm thick? You mean 20mm of the sealant? When you have stuff like that and it's thick it takes a lot longer to cure. Even though it's not sticky and feels dry, down inside of it can still be wet or just leaching acetic acid which is what you smell.
If you need to build up something that size it's best to have something inside as a "core" that way you only use a little silicone on the outside.
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