FRM-40010 cannot read form from /custom_top_dir/forms/US/xxxx.fmx
Is the issue with custom forms only?
Is "/custom_top_dir/forms/US/xxxx.fmx" a valid directory?
As applmgr user, what does "echo $<CUSTOM_TOP>" return after sourcing the application env file?
Is your CUSTOM_TOP set in the $APPL_TOP/custom<CONTEXT_NAME>.env and in the default.env file? -- Can't Open Custom Forms After R12 Upgrade (Doc ID 552010.1)
If you have to edit any of those env files please bounce the middle tier services and try then.
Many thanks following these steps resolved my issue!