I am shortly solved the problem based on this post:

https://moowrap.net/forum_thread.php?id=321&postid=5246#5246

But for ATI/AMD workunits i have to adjust the config saved as app_info.xml file
direct in your Windows "ProgramData\BOINC" folder.
Maybe u have to adjust this entry: <gpu_ram>2147483648</gpu_ram>

<app_info>
<app>
<name>dnetc</name>
<user_friendly_name>Distributed.net Client</user_friendly_name>
</app>
<file_info>
<name>dnetc_wrapper_1.5_windows_intelx86__opencl_ati_101.exe</name>
<executable/>
</file_info>
<file_info>
<name>dnetc521-win32-x86-opencl.exe</name>
<executable/>
</file_info>
<file_info>
<name>dnetc-gpu-1.5.ini</name>
</file_info>
<file_info>
<name>job-win32-opencl-521.1.xml</name>
</file_info>
<app_version>
<app_name>dnetc</app_name>
<version_num>102</version_num>
<platform>windows_intelx86</platform>
<avg_ncpus>0.050000</avg_ncpus>
<max_ncpus>0.895864</max_ncpus>
<plan_class>ati14</plan_class>
<flops>1157115231469.729200</flops>
<api_version>7.0.8</api_version>
<file_ref>
<file_name>dnetc_wrapper_1.5_windows_intelx86__opencl_ati_101.exe</file_name>
<main_program/>
</file_ref>
<file_ref>
<file_name>dnetc521-win32-x86-opencl.exe</file_name>
<copy_file/>
</file_ref>
<file_ref>
<file_name>dnetc-gpu-1.5.ini</file_name>
<open_name>dnetc.ini</open_name>
<copy_file/>
</file_ref>
<file_ref>
<file_name>job-win32-opencl-521.1.xml</file_name>
<open_name>job.xml</open_name>
<copy_file/>
</file_ref>
<coproc>
<type>ATI</type>
<count>1.000</count>
</coproc>
<gpu_ram>2147483648</gpu_ram>
</app_version>
</app_info>