Transaction failing with web service error

Hello Team,
We have encountered several transaction failures due to a web service error, with the resolution suggesting, "Ensure that the user executing the web service has the appropriate privileges in place before proceeding in the approval flow." Upon investigating the issue and reviewing the submitter's roles, we found that the roles are as expected (LPNT ESS & MSS roles). Additionally, we compared these roles with those of other submitters whose transactions were successfully routed and completed, noting that both sets of submitters have the same roles and privileges.

If anyone also faced similar issue & know the solution/root cause then please help us.