I get an error when I start Toad or try to connect to a database.

Index was outside the bounds of the array.
Stack Trace:
at System.Collections.ArrayList.ArrayListEnumeratorSimple.MoveNext()
at System.Data.Common.DbConnectionStringBuilder.get_ConnectionString()
at Quest.Toad.DB2.DB2Provider.ConnectString(ConnectMediator mediator)
at Quest.Toad.Db.Connection.get_ConnectString()
at Quest.Toad.DB2.DB2ToadConnection.CreateConnection()
at Quest.Toad.Db.Connection.AllocConnection()
at Quest.Toad.Db.Connection.Connect(Boolean notify)
at Quest.Toad.Db.Provider.BackgroundConnector.CreateBackgroundConnection()

I assume you are automatically connecting to a database at Toad startup which causes the product startup part of the issue.

Does this happen on all databases or only one database?

Is there something unusual about the target database alias name, userid, or password?