Yes, it will be hard to reproduce, since it requires accessing whatever non-thread-safe collection you have by several threads at the same time, but you should be able to mock this up. I’m attaching our stack trace of the location of the error from v20.12 in the hopes that will help you pinpoint the bug location. Can you please let me know if the non-thread safe collection at this location has been fixed?
System.InvalidOperationException: Operations that change non-concurrent collections must have exclusive access. A concurrent update was performed on this collection and corrupted its state. The collection’s state is no longer correct.
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at Wsl.PdfConverter.Implementation.PdfConverter.AddWatermarks(Byte[] inputPdfFileContent, IEnumerable`1 watermarks) in D:\PROJECTS\ncore-applicationserver-net_2021.1\WSL\PdfConverter\Implementation\PdfConverter.cs:line 524