Hello Dimitry,
In such case it might that your request is not built properly. RequestHeader must be there in your API request. Below is my sample request for CampaignService.get() API call for your reference. Please give it a try and let me know if you have any further questions.
[23 Oct 2019 12:58:34,232-soapXmlLogger:DEBUG:main] SOAP request:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<soapenv:Envelope xmlns:soapenv="
http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="
http://www.w3.org/2001/XMLSchema" xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance">
<soapenv:Header>
<ns1:RequestHeader xmlns:ns1="
https://adwords.google.com/api/adwords/cm/v201809" soapenv:mustUnderstand="0">
<ns1:clientCustomerId>876-052-7478</ns1:clientCustomerId>
<ns1:developerToken>REDACTED</ns1:developerToken>
<ns1:userAgent>GoogleAdsSupport (AwApi-Java, AdWords-Axis/4.6.0, Common-Java/4.6.0, Axis/1.4, Java/1.8.0_212, maven, SelectorBuilder, SelectorField)</ns1:userAgent>
<ns1:validateOnly>false</ns1:validateOnly>
<ns1:partialFailure>false</ns1:partialFailure>
</ns1:RequestHeader>
</soapenv:Header>
<soapenv:Body>
<get xmlns="
https://adwords.google.com/api/adwords/cm/v201809">
<serviceSelector>
<fields>Id</fields>
<fields>Name</fields>
<ordering>
<field>Name</field>
<sortOrder>ASCENDING</sortOrder>
</ordering>
<paging>
<startIndex>0</startIndex>
<numberResults>100</numberResults>
</paging>
</serviceSelector>
</get>
</soapenv:Body>
</soapenv:Envelope>