I too experienced a similar issue due to Visio being re-installed to resolve a font issue.
I highly recommend you work through the
document on the forums to resolve this issue. I had a complication with the process due to my error ( I ran the DTools LANSync application to create the cache file instead of stopping and starting the DTools LANSync v5 Server
service ).
This was my experience:
I believe our IT support rectified this font problem by re-installing Visio over the weekend.
I then worked through the ‘Troubleshooting SI 5 LANSync Errors/Messages’ document.
In order for DTools LANSync v5 Server service to run and not stop I needed to rename the cache file DToolsCache.xml (as per document).
I then ran the DTools LANSync application accidentally instead of the DTools LANSync v5 Server service stated in the document to create the cache file (this did not create the cache file).
I attempted to open SI5 on the local machine which generated the same errors.
I then noticed the cache file had not been recreated, and stopped and started the DTools LANSync v5 Server service (as per documents instruction) and this did create the cache file.
I then opened SI5 on the local machine, still the error appeared, I selected ignore option to error message.
SI5 navigator opened without requesting me to login.
I closed SI5 navigator.
Opened SI5 again with no error and logged in as per usual.