I am encountering this problem in version 5.0.1.480 with Oracle 8.1.7.4.1 on one database (only distinction of it from others known to me is that it is very large - 8 Gb) always, i.e. on any action (viewing tables list, reading body of a package, reading package, reading body of a trigger etc). On other databases all is ok. Tried to work with this database with Oracle 9i - I couldn't do anything too, but another error was encountered - don't remember its number, but text was something like "meaningless or inadmissible conversion". When I deinstalled PLSQL Developer 5 and installed PLSQL Developer 3 all was ok and I worked with database on Oracle 9i with pleasure.
Can you comment this behaviour?
For the present time I can't download mentioned patch, so can't say anything about its usability. Will try it after meeting sysadmins.