Some donors are entering currency prefixes inside the Other Amount field on the recurring upgrade form, which is preventing the form from being submitted. However, the form suggests the input is valid, resulting in donors reaching out to donor relations to see what's wrong.
We can fix this a couple of ways, such as stripping non-numeric data on submit or even adding a symbol prefix to the input field if that's enough.
This came to light in
T388912: Checksums and contact ids missing from recurring upgrade links and can be replicated locally
