Have the same issue (perhaps), looking at the debug text, noticed where it's closing the map on connection... if a Paradox/Introversion person can help.
Wife and I had been working on a large prison all day, her game crashed (she was host) and won't let me reconnect to it.
JoinRoomCallback success
Sent analytics event in 62ms. HTTP result was 200.
Sent analytics event in 65ms. HTTP result was 200.
Performance: Waited 0.000877 s or Update Thread to complete.
SaveGameSender: Expecting 28 chunks
SaveGameSender: Received chunk 0 size: 58981
Sent analytics event in 65ms. HTTP result was 200.
SaveGameSender: Received chunk 1 size: 58981
SaveGameSender: Received chunk 2 size: 58981
SaveGameSender: Received chunk 3 size: 58981
SaveGameSender: Received chunk 4 size: 58981
SaveGameSender: Received chunk 5 size: 58981
Sent analytics event in 70ms. HTTP result was 200.
SaveGameSender: Received chunk 6 size: 58981
SaveGameSender: Received chunk 7 size: 58981
SaveGameSender: Received chunk 8 size: 58981
SaveGameSender: Received chunk 9 size: 58981
SaveGameSender: Received chunk 10 size: 58981
Sent analytics event in 76ms. HTTP result was 200.
SaveGameSender: Received chunk 11 size: 58981
SaveGameSender: Received chunk 12 size: 58981
SaveGameSender: Received chunk 13 size: 58981
SaveGameSender: Received chunk 14 size: 58981
Sent analytics event in 54ms. HTTP result was 200.
SaveGameSender: Received chunk 15 size: 58981
SaveGameSender: Received chunk 16 size: 58981
SaveGameSender: Received chunk 17 size: 58981
SaveGameSender: Received chunk 18 size: 58981
SaveGameSender: Received chunk 19 size: 58981
Sent analytics event in 59ms. HTTP result was 200.
SaveGameSender: Received chunk 20 size: 58981
SaveGameSender: Received chunk 21 size: 58981
SaveGameSender: Received chunk 22 size: 58981
SaveGameSender: Received chunk 23 size: 58981
SaveGameSender: Received chunk 24 size: 58981
Sent analytics event in 53ms. HTTP result was 200.
Performance: Waited 0.002938 s or Update Thread to complete.
SaveGameSender: Received chunk 25 size: 58981
SaveGameSender: Received chunk 26 size: 58981
SaveGameSender: Received chunk 27 size: 50792
SaveGameSender: Finished receiving save game. Now decompressing. Size: 1643279
Sent analytics event in 60ms. HTTP result was 200.
Sent analytics event in 49ms. HTTP result was 200.
Performance: Waited 0.008317 s or Update Thread to complete.
Performance: Waited 0.009505 s or Update Thread to complete.
CompressionThread:

eserializeDirectory: Directory read failed.
MessageQueue AttractScreenThread (00000000605D77F0) running.
CLOSING MAP...
GridRoutingSystem(0000000027FFAFB0) destroyed.