Problem solved by installing SVE updates (SMAPI, Content Patcher, Extra Map Layers, Json Assets, Space Core) ...
... that's what I thought at least. I now can enter the building, talk with Martin, buy him a concession - but then cannot enter the cinema hall.
Still clueless ...
If it helps in any way, here is the error message:
[game] An error occurred in the base update loop: NullReferenceException: Object reference not set to an instance of an object.
at StardewValley.Locations.MovieTheater.GetFirstInvitedPlayer(NPC npc) in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Locations\MovieTheater.cs:line 1777
at StardewValley.Locations.MovieTheater.OnRequestStartMovieEvent(Int64 uid) in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Locations\MovieTheater.cs:line 485
at Netcode.AbstractNetEvent1`1.Poll() in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Netcode\NetEvent.cs:line 148
at StardewValley.Locations.MovieTheater._ShowMovieStartReady() in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Locations\MovieTheater.cs:line 578
at StardewValley.Locations.MovieTheater.OnMovieViewerLockEvent(MovieViewerLockEvent e) in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Locations\MovieTheater.cs:line 570
at Netcode.AbstractNetEvent1`1.Poll() in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Netcode\NetEvent.cs:line 148
at StardewValley.Locations.MovieTheater.<performAction>b__91_0() in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Locations\MovieTheater.cs:line 2035
at StardewValley.Network.NetMutex.Update(FarmerCollection farmers) in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Network\NetMutex.cs:line 89
at StardewValley.FarmerTeam.Update() in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\FarmerTeam.cs:line 1291
at StardewValley.Game1._update(GameTime gameTime) in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Game1.cs:line 4283
at StardewValley.Game1.Update(GameTime gameTime) in C:\GitlabRunner\builds\Gq5qA5P4\1\ConcernedApe\stardewvalley\Farmer\Farmer\Game1.cs:line 3974
at StardewModdingAPI.Framework.SCore.OnPlayerInstanceUpdating(SGame instance, GameTime gameTime, Action runUpdate) in SMAPI\Framework\SCore.cs:line 1117