I placed down a slab vertically using shift+click, the first time it crashed immediately, here is the log for that: first crash

i loaded back up the pack again, and suffered the same problem, however, i had an issue with it spamming crashes when i got close (vanillafix would tell me a problem had occured, but no crash). but then i jumped on it and it gave me the same crash

this seems entirely unique to the new rewrite of the mod, i will be using an older version until this is stable, thank you for your hard work!

https://github.com/TheidenHD/DoubleSlabs/releases/tag/0.11.1

Thank you for the bug report and sorry for the delayed response. I believe this is a duplicate of #140 :)

Just posting to say I had a similar crash when placing a normal slab vertically next to an identical vertical slab.
crash-2020-11-28_06.46.11-server.txt