Classic Shell 4.2.1 was developed for earlier builds of Windows 10 before build 10240 (the "RTM" build which is being distributed to computer makers). So it is not 100% compatible with the final Windows 10 version due to last minute changes by Microsoft.
Classic Shell 4.2.3 is compatible but again there were some updates released for Windows 10 apps due to which it started crashing. (one of the app's icon had a bad name). Until the next release of Classic Shell is out very soon, try this patch:
viewtopic.php?f=7&t=4822. Note that it is only for Classic Shell 4.2.3 and 64-bit Windows 10. The next version will incorporate this fix.
Note that Windows 10 itself is still buggy and updates to the OS can cause such incompatibilities.