Try to build the SOQL query (in an Assign activity) before the Search using SOQL.
Then try to use that variable in the activity.
Double check that variable to have the correct syntax before its use.
If is still not working, raise a support case.
maybe you can also try
“Select Id from Account where Customer_number_c = ‘“+Customer_number+””
instead of
“Select Id from Account where Customer_number_c = ‘<Customer_number>’”