This happens due to the files being created/read/moved by the plugin on startup.

eg (this happens over and over again on startup):

stat("/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-36", {st_mode=S_IFREG|0600, st_size=1262, ...}) = 0
rename("/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-36", "/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-36.bak") = 0
rename("/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-36.tmp", "/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-36") = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/usr/share/zoneinfo/UTC", {st_mode=S_IFREG|0644, st_size=118, ...}) = 0
open("/usr/share/zoneinfo/UTC", O_RDONLY|O_CLOEXEC) = 18
fstat(18, {st_mode=S_IFREG|0644, st_size=118, ...}) = 0
fstat(18, {st_mode=S_IFREG|0644, st_size=118, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f98fbe17000
read(18, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1\0\0\0\1\0\0\0\0"..., 4096) = 118
lseek(18, -62, SEEK_CUR)                = 56
read(18, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\1\0\0\0\1\0\0\0\0"..., 4096) = 62
close(18)                               = 0
munmap(0x7f98fbe17000, 4096)            = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
open("/etc/localtime", O_RDONLY|O_CLOEXEC) = 18
fstat(18, {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
fstat(18, {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f98fbe17000
read(18, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\4\0\0\0\4\0\0\0\0"..., 4096) = 2294
lseek(18, -1457, SEEK_CUR)              = 837
read(18, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\4\0\0\0\4\0\0\0\0"..., 4096) = 1457
close(18)                               = 0
munmap(0x7f98fbe17000, 4096)            = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2294, ...}) = 0
access("/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-37", W_OK) = 0
open("/home/dglidden/.claws-mail/vcalendar/040000008200E00074C5B7101A82E00800000000A06CDE125E10CE01000000000000000010000000FED546FF4897274DAFAB7C0F5CD85DCC-37.tmp", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 18