Changes in Oracle Database implicit conversion rules for select statement
So you have a VARCHAR column, and a NUMBER filter. The implicit conversion indeed allows a NUMBER -> VARCHAR conversion, but the problem is that when applying the filter, Oracle tries to do it the other way, ie. to convert the VARCHAR column into a number, thus the error gets thrown :) 

Don't really see any solutions here except updating the SQL query. This is why nobody recommends the usage of implicit conversions. They could also cause a slight decrease of performance.

EDIT: If you don't have access to the query and have to wait for the dev team, but instead you have enough privileges on the database, and know for sure that the VARCHAR column holds only number, you could try to alter the table and convert the column type.