Found the following entries in my error log this morning. I'm pretty sure these occurred upon opening MB as I don't think I was yet playing anything at the time they were thrown.
4/19/2018 12:30:20 AM - 10.0.16299.0 - 3.2.6665.34905 - System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at System.Collections.Generic.List`1.Enumerator.MoveNext()
at #=zfVOWnuz6Wh$HaskOtzmfiedeH9NK.#=ztizM69awK7T3(Boolean #=z1ch4PUrz3iUf, SortOrder #=zeks4fPAvS$yoHmsdNA==)
at #=zfVOWnuz6Wh$HaskOtzmfiedeH9NK.#=zhnpL8w8=(#=zpnkmxwY4IX__pZjMupoooME= #=z9P49bjQ=)
at #=z4$90odPQW5q9S2kLFw==.#=zIxnsWjiZbPJh(String #=zhNzA16PTIpIO, String #=z_x8_vbgH6Yw8)
at MusicBeePlugin.AdvancedSearchAndReplacePlugin.AutoApply(Object currentFileObj, Object tagToolsPluginObj)
at MusicBeePlugin.Plugin.ReceiveNotification(String sourceFileUrl, NotificationType type)
--- End of inner exception stack trace ---
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at #=z4$90odPQW5q9S2kLFw==.#=zIQ$i_ckXhk3p()
4/19/2018 12:30:21 AM - 10.0.16299.0 - 3.2.6665.34905 - System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
at System.Collections.Generic.List`1.Enumerator.MoveNext()
at #=zfVOWnuz6Wh$HaskOtzmfiedeH9NK.#=ztizM69awK7T3(Boolean #=z1ch4PUrz3iUf, SortOrder #=zeks4fPAvS$yoHmsdNA==)
at #=zfVOWnuz6Wh$HaskOtzmfiedeH9NK.#=zhnpL8w8=(#=zpnkmxwY4IX__pZjMupoooME= #=z9P49bjQ=)
at #=z4$90odPQW5q9S2kLFw==.#=zIxnsWjiZbPJh(String #=zhNzA16PTIpIO, String #=z_x8_vbgH6Yw8)
at MusicBeePlugin.AdvancedSearchAndReplacePlugin.AutoApply(Object currentFileObj, Object tagToolsPluginObj)
at MusicBeePlugin.Plugin.ReceiveNotification(String sourceFileUrl, NotificationType type)
--- End of inner exception stack trace ---
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at #=z4$90odPQW5q9S2kLFw==.#=zIQ$i_ckXhk3p()
Don't know if it's related, but when I opened MB this morning it immediately locked up without loading the library or anything else (last.fm icon at the bottom was "!" for example). I had to "End Task" it in the Task Manager and then it worked fine when I reopened it.
EDIT: Looking back, I see this error kind of frequently in my error log going back to January 2018 when I was using MB version 3.1.6596.