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It has to deal with the security tokens, likely. Try browsing to the page you want to change, or do a hard refresh of the page before submitting your changes. Either will generate a new security token for that process. The token will get "used" when you submit the changes (or expire if you don't). If you use the browser back button to backup to that page and try to submit it again (with or without changes), that associated security token will no longer be valid, so must be regenerated before allowing such action to occur.
Does that explain your process to reproduce this error? If not, I might suggest that you explain the process you used to reproduce the error so others may attempt the same process; also useful to say what version of XOOPS you're using (assuming latest of 2.3.3).
P.S. "Very strange error" isn't a very descriptive subject line for this issue, especially since it's really not that strange
![](https://xoops.org/uploads/smil42e7a65fee35d.gif)
. Although attention-grabbing, better might be "Error Changing System Permissions" (or whatever appropriate).