Problem: Sometimes, especially with TortoiseSVN on Windows, the svn cleanup command doesn't work and the suggested fix is to run svn cleanup. There seems to be no escaping from this except to check out a fresh copy, which for big repositories would be a major headache and also time-consuming.
Solution: The problem is one or more unfinished operations in SVNs work queue, which is stored in a SQLite3 database in the file .svn/wc.db at the top-level directory of the working copy of the repository. The following two commands do the following: the first command shows the list of pending operations (not strictly necessary, but good to check, whether only the one is shown, which one actually wants to cancel) and the second command deletes all these operations:
sqlite3.exe .svn/wc.db "select * from work_queue" sqlite3.exe .svn/wc.db "delete from work_queue"
Of course, this requires that SQLite3 is installed. On Ubuntu, this is as simple as sudo apt install sqlite3. On Windows, a binary can be downloaded from the SQLite download site (pick the one with the command-line tools included, because what you one is the sqlite3.exe executable): https://www.sqlite.org/download.html