I debugged this issue today (although I was sure the backend was OpenSSL, huh), the ultimate problem is that SOGo

a) fails to call SSL_load_error_strings(), and

b) assumes ERR_reason_error_string() can never return NULL (but it can, for unknown error codes). When SSL_load_error_strings() wasn't called, a lot of error codes are unknown, but I still wouldn't assume /every/ code to be valid even if it was called.

The NULL returned from ERR_reason_error_string() is passed to NSString's "stringWithUTF8String"-constructor which then fires an NSInvalidArgumentException (which makes it into the logfile).

Here's my patch that fixes the issue:

--- sogo-4.0.0.orig/UI/MailPartViewers/UIxMailPartSignedViewer.m
+++ sogo-4.0.0/UI/MailPartViewers/UIxMailPartSignedViewer.m
@@ -170,8 +170,9 @@
if (err)
{
ERR_load_crypto_strings();

• SSL_load_error_strings();
  sslError = ERR_reason_error_string(err);
• validationMessage = [[self labelForKey: [NSString stringWithUTF8String: sslError]] retain];
• validationMessage = [[self labelForKey: [NSString stringWithUTF8String: sslError ? sslError : "(no error information available)"]] retain];
  }
  }

Edit: Oh well. Uploaded the patch as an attachment, too.