Hello Nicola,
"Assertion failed" errors come out when configurations are different from what they should be. According to Note 1977393the setup of Oracle RAC is now integrated in SWPM, probably in your scenario some steps were done manually.
The best option is to follow Note 1977393 and the installation guide, there are specific sections for Oracle ASM. To make an example, oracle user is created by SWPM.
Regards,
Valerio