Hi Lóránd,
The range of acceptable bid modifier ranges can be found in this
documentation. That being said, if the value that you specified is within the ranges stated in the documentation, you may send to me the complete SOAP request and response logs so I can further investigate. If you haven't enabled SOAP logging yet, let me know which client library you are using so I can provide you the appropriate guide to enable them.
Once the SOAP logs are available, you can send the details I requested via the
Reply privately to author option.
Thanks and regards,
Peter
Google Ads API Team

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