Symptoms
An API request is sent, one of the parameter values passed has an ampersand symbol (&). API result is unexpected - the system considers the values as empty, e.g.:
parser error : xmlParseEntityRef: no name (for AccountAdd_API)
"Password cannot be empty" (for UserForVendorValidate_API)
Cause
Ampersand must be encoded when passed to xml, that is mentioned in the documentation.
Resolution
Use & instead of &.
Note, the other symbols, e.g. <, >,'," also must be encoded.