Apologies.
1) It was in version V10.13.18 while playing a single level (selected from the dialog box upon running the program)
2) While playing the level.
3) I have the default hotkey assigned to F4, which selects the Floater, but for me, this crash occurs even if the level in question doesn't have any Floaters.
Error message:
Exception EAccessViolation in module NeoLemmix.exe at 00F8023.
Access Violation at address 008F8023 in module 'NeoLemmix.exe'. Read of address 00DC80DC.
Notably, the first two hexadecimal numbers are always the same, but the read of address is different each time.