http://www.opcsupport.com/link/portal/4164/4590/ArticleFolder/51/Citect
To decode the error messages carry out the following steps:
*Open up Windows Calculator (Start > Run > calc)
*Switch to scientific mode (View > Scientific).
*Select Decimal mode (Dec).
*Type or copy and paste the error number into calculator e.g. 268435730.
*Switch to Hexadecimal mode (Hex). Example result is 10000112.
*Subtract 10000000 (1 followed by 7 x 0‘s) from the number to give the CTAPI error code in Hex (e.g. 112).
*Switch back to Decimal mode (Dec) to get the CTAPI error code in decimal (e.g. 274).
*Look up the decimal error code in the Citect help file (Citect.hlp) or Citect Knowledge Base (ctkbase.chm). The attached zip file contains some versions of these files from an older version of Citect. This step is a bit hit and miss and requires a bit of searching around. To assist with this, the following articles/pages are of most use:
*Help file - Protocol Driver Errors page (in the help file select Index > Index tab and type "Protocol Driver Errors" or "driver" to locate the page).
*Help file - Hardware/Cicode Errors (in the help select Index > Index tab and type Hardware and select the "Error messages" topic under the Hardware Alarms section).
*Citect KB - search the KB for the error code. This works for error 274 - "username or password is incorrect".
Additional information:
0x10000000 = 268435456 = The Citect
ERROR_USER_DEFINED_BASE. Error codes returned by the CTAPI that are less
than ERROR_USER_DEFINED_BASE are Microsoft Win32 error codes. Error
codes greater than ERROR_USER_DEFINED_BASE are CitectHMI/SCADA special
error codes (hence we subtract ERROR_USER_DEFINED_BASE from the original
error code to get the CTAPI special error code).
*The "CTAPI Error Codes" topic in the Citect help file provides
information on the CTAPI error codes and how to decode them. To locate
the article in the help select Index > Index tab and type "CTAPI:"
and select the "CTAPI Error Codes" topic under the CTAPI section.
*The topic number Q4126 - "CTAPI connection fails due to incorrect
user name or password" in the Citect KB file provides information on
error code 247 - "username or password is incorrect".
Some common error codes and their corresponding messages:
*268435730 = 0x10000112 = 274 - "username or password is incorrect".
*268435745 = 0x10000121 = 289 - "blank password"
*268435481 = 0x10000019 = 25 - "Data not yet valid"
*268435468 = 0x1000000C = 12 - "I/O Device off-line, cannot talk"
*268435879 = 0x100001A7 = 423 - "Subscription not yet ready"
Here is a larger list for Citect V7.0 onwards - 
Citect V7.0 error codes
----------------------------
Update! 06MAR13RF - 289 (blank password) can be sent back when
the user or password in the Citect project is incorrect. If you get
this error and you have defined a password, and Citect v7 or above is
used, please check that you can log in to the Citect project with the
username and password that are being used in the OPC Server.