We have a Power App canvas app that uses a form for item Adds & Edits (same form, different mode). Users are encountering the error when saving via SubmitForm(<formname>), but only on some records, about 4 out of ~4000 so far. Once encountered, the item will not save any changes, which leads us to believe it is not a random connectivity issue. As it appears only related to specific records, we've reviewed those items to ensure no bad/invalid data exists in a complex or non-text data type column and found no oddities. If we were writing out a read-only field on the form, we expect every record would have this issue. The form does display (read-only) some additional lookup field data, but that have been there since inception and never led to any for save issues. To date:
The error does not indicate WHICH field is read-only. My next attempt is to recreate the form in another app and see if those particular records will save.
Open to any thoughts or suggestions as the error message is not all that helpful.
RESOLVED: The issue was users entering invalid lookup values for a particular field. The App was letting them create items with an invalid value, but prevented those items from saving on edit. The fix was to add the needed values to the lookup list and then update the missing value in the item. Both were required to get by the Network Error. We had modified this particular datacards input field from a ComboBox to text input which made this possible.
Data source for the above is SharePoint Online. The source and form are quite large (150+ columns). App has been in use for 9 months. Problem occurred only recently.