I'm glad you've been enjoying the game and thank you for helping to uncover these issues!
The first crash must have happened when the game ran out of resources for allocating more blocks during the endless mode, Skyland can only spawn 181 blocks at once. I've never hit the upper limit myself, but it definitely seems to be possible, so I'll try to increase it.
I fixed the second crash, and yeah that's indeed what's breaking the game. The game will sometimes give you some free terrain when picking up blocks from quests, various levels, etc. if your island is full to prevent you from having to salvage stuff. While you can't build an island larger than 14 blocks wide under normal circumstances, 16 blocks wide is a hard limit on the size of an island, the game got stuck in an infinite loop trying and failing to increase the size of an island past 16 until it eventually crashed due to a separate problem.
<https://github.com/evanbowman/skyland-beta/releases/tag/2026.8.11.0>