SQL Server - AppDomain is marked for unload due to memory pressure

On SQL Server Error Log messages I see this error message relating to memory pressure:

AppDomain xxxxxxxxxxxxxxxx is marked for unload due to memory pressure.
AppDomain xxxxxxxxxxxxxxxx is marked for unload due to common language runtime (CLR) or security data definition language (DDL) operations.

Some important points to consider:

1) SQLCLR runs in the SQLOS in limited memory space called MemToLeave.A number of processes run inside the MemToLeave – such as OLE automation and 3rd party backup applications. For a comprehensive list check: SQL server OLE Automation memory pressure - SQL Server DBA

2) Investigate other applications running on the server. Huge memory bottlenecks can occur with external applications on the server.

3) Look at server memory and SQL Server memory. Is there enough memory?

4) Use these Views for informations on assemblies and application domains on the server

sys.assemblies
sys.dm_clr_appdomains
sys.dm_clr_loaded_assemblies

5) Watch the memory problem on:

select * from sys.dm_os_memory_clerks where type = 'MEMORYCLERK_SQLCLR'

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