2015-07-22 18:40:38: ERROR[main]: UNRECOVERABLE error occurred. Stopping server. Please fix the following error:
2015-07-22 18:40:38: ERROR[main]: ...inetest/minetest/bin/../builtin/common/serialize.lua:152: Can't serialize data of type userdata
2015-07-22 18:40:38: ERROR[main]: stack traceback:
2015-07-22 18:40:38: ERROR[main]: 	[C]: in function 'error'
2015-07-22 18:40:38: ERROR[main]: 	...inetest/minetest/bin/../builtin/common/serialize.lua:152: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:115>
2015-07-22 18:40:38: ERROR[main]: 	(tail call): ?
2015-07-22 18:40:38: ERROR[main]: 	...inetest/minetest/bin/../builtin/common/serialize.lua:147: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:115>
2015-07-22 18:40:38: ERROR[main]: 	(tail call): ?
2015-07-22 18:40:38: ERROR[main]: 	...inetest/minetest/bin/../builtin/common/serialize.lua:147: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:115>
2015-07-22 18:40:38: ERROR[main]: 	(tail call): ?
2015-07-22 18:40:38: ERROR[main]: 	...inetest/minetest/bin/../builtin/common/serialize.lua:167: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:20>
2015-07-22 18:40:38: ERROR[main]: 	(tail call): ?

In thread 7ff801228740:
/home/minetest/minetest/src/server.cpp:505: void Server::step(float): A fatal error occurred: ...inetest/minetest/bin/../builtin/common/serialize.lua:152: Can't serialize data of type userdata
stack traceback:
	[C]: in function 'error'
	...inetest/minetest/bin/../builtin/common/serialize.lua:152: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:115>
	(tail call): ?
	...inetest/minetest/bin/../builtin/common/serialize.lua:147: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:115>
	(tail call): ?
	...inetest/minetest/bin/../builtin/common/serialize.lua:147: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:115>
	(tail call): ?
	...inetest/minetest/bin/../builtin/common/serialize.lua:167: in function <...inetest/minetest/bin/../builtin/common/serialize.lua:20>
	(tail call): ?
Debug stacks:
DEBUG STACK FOR THREAD 7ff7f9fd1700:
#0  virtual void* EmergeThread::Thread()
(Leftover data: #1  MapBlock* ServerMap::loadBlock(v3s16))
(Leftover data: #2  void ServerMap::loadBlock(std::string*, v3s16, MapSector*, bool))
(Leftover data: #3  void ServerMap::loadBlock(std::string*, v3s16, MapSector*, bool))
DEBUG STACK FOR THREAD 7ff7fa7d2700:
#0  virtual void* ServerThread::Thread()
#1  void Server::Receive()
(Leftover data: #2  void Server::SendBlocks(float))
(Leftover data: #3  void Server::SendBlockNoLock(irr::u16, MapBlock*, irr::u8, irr::u16))
(Leftover data: #4  MapBlock* ServerMap::loadBlock(v3s16))
(Leftover data: #5  void ServerMap::loadBlock(std::string*, v3s16, MapSector*, bool))
DEBUG STACK FOR THREAD 7ff801228740:
#0  int main(int, char**)
#1  Dedicated server branch
#2  void dedicated_server_loop(Server&, bool&)
#3  void Server::step(float)