Something is a bit off with boincmgr on Ubuntu, as sometimes it will not start. I ran an strace on it and found the right file to delete, but you might fix the code so it can start more reliably. Here is the tail of the strace:

19054 openat(AT_FDCWD, "/home/dgp/BOINC Manager-dgp", O_WRONLY|O_CREAT|O_EXCL, 0600) = -1 EEXIST (File exists)
19054 stat("/home/dgp/BOINC Manager-dgp", {st_dev=makedev(8, 19), st_ino=262181, st_mode=S_IFREG|0600, st_nlink=1, st_uid=1000, st_gid=1000, st_blksize=4096, st_blocks=8, st_size=5, st_atime=1590069452 /* 2020-05-21T09:57:32.092720457-0400 */, st_atime_nsec=92720457, st_mtime=1588288197 /* 2020-04-30T19:09:57.057893589-0400 */, st_mtime_nsec=57893589, st_ctime=1588288197 /* 2020-04-30T19:09:57.061893668-0400 */, st_ctime_nsec=61893668}) = 0
19054 getuid() = 1000
19054 openat(AT_FDCWD, "/home/dgp/BOINC Manager-dgp", O_RDONLY) = 10
19054 read(10, "3705\0", 256) = 5
19054 kill(3705, SIG_0) = 0
19054 close(10) = 0
19054 getpid() = 19054
19054 getpid() = 19054
19054 exit_group(-1) = ?
19056 <... poll resumed> <unfinished ...>) = ?
19055 <... poll resumed> <unfinished ...>) = ?
19056 +++ exited with 255 +++
19055 +++ exited with 255 +++
19054 +++ exited with 255 +++