Well I tried to save a multiplayer map, with custom blocks and load it in singleplayer, but it crashes with this:
Crash time: 02/07/2021 14:23:57
ClassiCube crashed.
Reason: Unhandled exception 0xC00000FD at 0x000000000041FE23 [0x0000000000000001,0x0000000000713FF8,]
Commit SHA: 0421d68
-- registers --
rax=0000000000000029 rbx=0000000000000004 rcx=00000000000000A0
rdx=0000000000000029 rsi=0000000000000049 rdi=000000000669F040
rip=000000000041FE23 rbp=000000000117BA00 rsp=0000000000714000
r8 =00000000000000A4 r9 =000000000000000C r10=000000000003D090
r11=000000000000000C r12=000000000000009F r13=0000000000000030
r14=000000000000003F r15=00000000003955CB
-- backtrace --