IOERROR: ... Interrupted system call

[brian]

I'm seeing the following in the log:

IOERROR: locking header for DOMAIN.TLD!user.admin.Sent: Interrupted 
system call

and:

IOERROR: locking /PATH/TO/user/a/admin.seen: Interrupted system call

Sometimes it's "lock_shared" instead of "locking". I'm seeing variations 
of these for all 5 mailboxes. (Yeah, not a big operation--a client's 
business server.)

The users are reporting that Thunderbird, which they all use, appears to 
hang sometimes when copying to the Sent folder. Also, some messages are 
supposedly not arriving at their destination. I presume that's because 
they're canceling from the client but, if the trouble has to do with 
just copying to Sent, I can't see how that would affect the original 
message delivery. I'm awaiting further clarification on that.

Meanwhile, I can see from the log that something's definitely not right. 
I've been searching online and have only found a very few posts about 
this. But those all seem to be related to 2.1.x and, in any case, I 
cannot find any information that conclusively describes either the 
problem or the fix. I'd very much appreciate any suggestions.

I'm running 2.2.13-14ubuntu3.1 (yes, Debian packages are ridiculously 
outdated.) I'd be happy to compile a newer version from source if I knew 
for sure that this is a problem that's since been fixed.