I've checked our files and haven't seen any open investigations the same as yours, Userbukuria-ssr. I'll help you fix it.
I've also replicated it on my end, and I was able to create an expense transaction without any issues. The unexpected behavior can be caused by too much cache. We'll perform some basic troubleshooting to fix this. Let's start with logging into your account through a private browser. It's the best place to identify browser issues. These are the shortcut keys:
Google Chrome: press Ctrl + Shift + N
Mozilla Firefox: press Ctrl + Shift + P
Safari: press Command + Shift + N
If you can create an expense transaction using the private browsing session, go back to your regular browser and clear its cache to start fresh.
On the other hand, you can use a different browser to narrow down the issue.