I am running into this same problem. Was there ever a solution found? It almost certainly feels like a bug because if the bank is automatically loaded, then my load and unload calls work. Once the bank is unloaded, these errors occur using the same functions and arguments and the bank can't be loaded and unloaded. The returned error is therefore incorrect since it can locate and unload the bank if the bank was automatically loaded prior.
When bank was previously auto-loaded by Wwise:
"
Bank Load Request Received (from [...].bnk)
Bank Already Loaded (from [...])
Bank Unload Request Received (from [...].bnk)
Bank Unloaded (from [...])
"
Then, running the exact same functions and arguments:
"
Bank Load Request Received (from [...].bnk)
Cannot open file [...].bnk in path(s):
Bank Load Failed
Bank Load Failed (from [...])
Bank Unload Request Received (from [...].bnk)
Unload bank failed, requested bank was not found: [...].bnk
"
Our Unreal Integration is correctly pointing to the WwiseProject, the GeneratedSoundBanks folder of the WwiseProject and the correct Wwise Installation version. Additionally, when the above errors occur, it shows the bnk under the "Object Name" column in the profiler alongside the request messages, indicating that it has indeed located the bank but is failing to load and unload it and not giving useful information why.