I faced that warning message during my restore process from RAC Database with ASM that has been converted or restored to the Standalone Single Instance Non-ASM Database.

I also noticed RMAN is taking significant time than normal restore time.

From the RDBMS alert.log, it is showing continuous following messages.

ASMB started with pid=32, OS id=28124 
WARNING: failed to start ASMB (connection failed) state=0x1 sid=''
WARNING: ASMB exiting with error
Starting background process ASMB

After investigating the issue I found MOS  2138520.1&2081537.1

This is due to some old references to files in the ASM but should not affect the functioning of the database.

You may ignore this message or you can apply path 19503821 for your environment.

Reference:
WARNING: failed to start ASMB after RAC Database on ASM converted to Single Instance Non-ASM Database (文档 ID 2138520.1)
12c RMAN Operations from ASM To Non-ASM Slow Performance (文档 ID 2081537.1)