Presumably in order to adhere to new gambling rules pertaining to loot boxes, players in the Netherlands and Belgium are now blocked from opening key-locked boxes in Counter-Strike: Global Offensive. This is following their announcement last month, where CS:GO and Dota 2 market trading was blocked for Dutch players (as covered here by Eurogamer), with promises that a "less inconvenient" solution would be coming.
While loot boxes are now forever-sealed for Belgian & Dutch players, the previous Netherlands-exclusive restriction on item trading has been lifted, according to yesterday's patch notes:
"Updated Steam and CS:GO account restrictions for users in Netherlands and Belgium:"
"Steam Trading and Steam Market features are now re-enabled for Steam accounts in Netherlands."
"Customers in Netherlands and Belgium will be restricted from opening containers"
While opening loot boxes is restricted for Belgian and Netherlands-based accounts, players can purchase the contents of other people's loot boxes directly through the Steam Market - ironically a key part of the equation that caused the Belgian Gaming Commission to declare loot boxes an exercise in gambling. While buying direct is a reasonable proposition for the majority of CS:GO skins, it does mean that rare and super-expensive liveries are now likely to be the sole domain of comically wealthy players (or those who got in pre-restriction) in these two countries.
The Belgian Gaming Commission's decision does not apply to all loot boxes, but rather ones specifically adhering to a set of rules closely enough to consider it gambling. There needs to be a bet that can lead to profit or loss, as well as an element of chance. In Valve's case, you paid £1.79/$2 per crate unlock key (which sounds like a bet to me), you could end up with an item far less or more valuable than the key paid for it, and the unofficial skin-trading market provides a route to cash out. Sounds like gambling.
At present, Valve's other crate-filled games - Team Fortress 2 and Dota 2 - seem unaffected (at least according to their recent patch notes), although that may change. They operate under a fundamentally identical system to Counter-Strike, although perhaps have flown under the radar as they've sparked less gambling-related controversy. At least for the time being, Belgians can still acquire the Belgian Detective hat the way God (and Valve) intended: Through blind luck.