sp_readerrorlog needs to open a new connection to get to the data. You are in single user mode and are already using the only connection available.
You can always read the error log using some other tool, such as Notepad.
Original author: SQL Server FineBuild 1-click install and best practice configuration of SQL Server 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005. 1 Dec 2016
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Quote: "When I give food to the poor they call me a saint. When I ask why they are poor they call me a communist." - Archbishop Hélder Câmara