Help needed - can't open html files on TyTNII

[Guest Restorer]

Suddenly I find I cannot open any html files from the device. PIE, Opera mini, File Explorer and Resco File Explorer all give the same error "The file "xxxx" cannot be opened. Either it is not signed with a trusted certificate or one of its components cannot be found". Same happens with apps which have to access offline html files. Also can't I get to my Orange account as this seems to rely on first opening an offline file.

Works fine online - just affects files stored on the device or storage card. Hope someone can help.